perf tools: Fix build breakage if prefix= is specified

Invoking Makefile.perf with prefix= breaks the build since Makefile.perf
hands that variable down to Makefile.build where it overrides

    prefix       := $(subst ./,,$(OUTPUT)$(dir)/)

leading to errors like this:

    No rule to make target '/usrabspath.o', needed by '/usrlibperf-in.o'
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Fixes: c819e2cf
Link: http://lkml.kernel.org/r/5582c48a.84a22b0a.a918.5285SMTPIN_ADDED_MISSING@mx.google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/build/Makefile.build

@@ -94,12 +94,12 @@ obj-y        := $(patsubst %/, %/$(obj)-in.o, $(obj-y))
 subdir-obj-y := $(filter %/$(obj)-in.o, $(obj-y))
 
 # '$(OUTPUT)/dir' prefix to all objects
-prefix       := $(subst ./,,$(OUTPUT)$(dir)/)
-obj-y        := $(addprefix $(prefix),$(obj-y))
-subdir-obj-y := $(addprefix $(prefix),$(subdir-obj-y))
+objprefix    := $(subst ./,,$(OUTPUT)$(dir)/)
+obj-y        := $(addprefix $(objprefix),$(obj-y))
+subdir-obj-y := $(addprefix $(objprefix),$(subdir-obj-y))
 
 # Final '$(obj)-in.o' object
-in-target := $(prefix)$(obj)-in.o
+in-target := $(objprefix)$(obj)-in.o
 
 PHONY += $(subdir-y)