docs: kbuild: fix invalid ReST syntax

I see the following warnings when I open this document with a ReST
viewer, retext:

/home/masahiro/ref/linux/Documentation/kbuild/makefiles.rst:1142: (WARNING/2) Inline emphasis start-string without end-string.
/home/masahiro/ref/linux/Documentation/kbuild/makefiles.rst:1152: (WARNING/2) Inline emphasis start-string without end-string.
/home/masahiro/ref/linux/Documentation/kbuild/makefiles.rst:1154: (WARNING/2) Inline emphasis start-string without end-string.

These hunks were added by commit e846f0dc ("kbuild: add support
for ensuring headers are self-contained") and commit 1e21cbfa
("kbuild: support header-test-pattern-y"), respectively. They were
written not for ReST but for the plain text, and merged via the
kbuild tree.

In the same development cycle, this document was converted to ReST
by commit cd238eff ("docs: kbuild: convert docs to ReST and rename
to *.rst"), and merged via the doc sub-system.

Merging them together into Linus' tree resulted in the current situation.

To fix the syntax, surround the asterisks with back-quotes, and
use :: for the code sample.

Fixes: 39ceda5c ("Merge tag 'kbuild-v5.3' of git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Documentation/kbuild/makefiles.rst

@@ -1152,7 +1152,7 @@ When kbuild executes, the following steps are followed (roughly):
 
     header-test-y
 
-	header-test-y specifies headers (*.h) in the current directory that
+	header-test-y specifies headers (`*.h`) in the current directory that
 	should be compile tested to ensure they are self-contained,
 	i.e. compilable as standalone units. If CONFIG_HEADER_TEST is enabled,
 	this builds them as part of extra-y.
@@ -1160,11 +1160,11 @@ When kbuild executes, the following steps are followed (roughly):
     header-test-pattern-y
 
 	This works as a weaker version of header-test-y, and accepts wildcard
-	patterns. The typical usage is:
+	patterns. The typical usage is::
 
-		  header-test-pattern-y += *.h
+		header-test-pattern-y += *.h
 
-	This specifies all the files that matches to '*.h' in the current
+	This specifies all the files that matches to `*.h` in the current
 	directory, but the files in 'header-test-' are excluded.
 
 6.7 Commands useful for building a boot image