fix typo "is" -> "if" in Makefile

It should be "if" but is written as "is"..
Signed-off-by: Uwe Kleine-Koenig <Uwe.Kleine-Koenig@digi.com>
Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>

Makefile

@@ -1538,7 +1538,7 @@ quiet_cmd_rmdirs = $(if $(wildcard $(rm-dirs)),CLEAN   $(wildcard $(rm-dirs)))
 quiet_cmd_rmfiles = $(if $(wildcard $(rm-files)),CLEAN   $(wildcard $(rm-files)))
       cmd_rmfiles = rm -f $(rm-files)
 
-# Run depmod only is we have System.map and depmod is executable
+# Run depmod only if we have System.map and depmod is executable
 # and we build for the host arch
 quiet_cmd_depmod = DEPMOD  $(KERNELRELEASE)
       cmd_depmod = \