kbuild: fix 'No such file or directory' warning when cleaning

Since commit fcbb8461 ("kbuild: remove header compile test"),
'make clean' with O= option in the pristine source tree emits
'No such file or directory' warning.

$ git clean -d -f -x
$ make O=foo clean
make[1]: Entering directory '/home/masahiro/linux/foo'
find: ‘usr/include’: No such file or directory
make[1]: Leaving directory '/home/masahiro/linux/foo'

Fixes: fcbb8461 ("kbuild: remove header compile test")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

usr/include/Makefile

@@ -91,7 +91,7 @@ endif
 # asm-generic/*.h is used by asm/*.h, and should not be included directly
 header-test- += asm-generic/%
 
-extra-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h'))
+extra-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/dev/null))
 
 quiet_cmd_hdrtest = HDRTEST $<
       cmd_hdrtest = \