selftests: vm: fix clang build error multiple output files

When building the vm selftests using clang, some errors are seen due to
having headers in the compilation command:

  clang -Wall -I ../../../../usr/include  -no-pie    gup_test.c ../../../../mm/gup_test.h -lrt -lpthread -o .../tools/testing/selftests/vm/gup_test
  clang: error: cannot specify -o when generating multiple output files
  make[1]: *** [../lib.mk:146: .../tools/testing/selftests/vm/gup_test] Error 1

Rework to add the header files to LOCAL_HDRS before including ../lib.mk,
since the dependency is evaluated in '$(OUTPUT)/%:%.c $(LOCAL_HDRS)' in
file lib.mk.

Link: https://lkml.kernel.org/r/20220304000645.1888133-1-yosryahmed@google.com
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/vm/Makefile

@@ -1,6 +1,8 @@
 # SPDX-License-Identifier: GPL-2.0
 # Makefile for vm selftests
 
+LOCAL_HDRS += $(selfdir)/vm/local_config.h $(top_srcdir)/mm/gup_test.h
+
 include local_config.mk
 
 uname_M := $(shell uname -m 2>/dev/null || echo not)
@@ -140,10 +142,6 @@ endif
 
 $(OUTPUT)/mlock-random-test $(OUTPUT)/memfd_secret: LDLIBS += -lcap
 
-$(OUTPUT)/gup_test: ../../../../mm/gup_test.h
-
-$(OUTPUT)/hmm-tests: local_config.h
-
 # HMM_EXTRA_LIBS may get set in local_config.mk, or it may be left empty.
 $(OUTPUT)/hmm-tests: LDLIBS += $(HMM_EXTRA_LIBS)