usr/include: replace extra-y with always-y

extra-y is not run for 'make modules'. The header compile test should
be executed irrespective of the build target. always-y is a better fit.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

usr/include/Makefile

@@ -95,7 +95,7 @@ endif
 # asm-generic/*.h is used by asm/*.h, and should not be included directly
 no-header-test += asm-generic/%
 
-extra-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/dev/null))
+always-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/dev/null))
 
 # Include the header twice to detect missing include guard.
 quiet_cmd_hdrtest = HDRTEST $<