kbuild: fix comment about missing include guard detection

The keyword here is 'twice' to explain the trick.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

usr/include/Makefile

@@ -93,7 +93,7 @@ no-header-test += asm-generic/%
 
 extra-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/dev/null))
 
-# Include the header to detect missing include guard.
+# Include the header twice to detect missing include guard.
 quiet_cmd_hdrtest = HDRTEST $<
       cmd_hdrtest = \
 		$(CC) $(c_flags) -S -o /dev/null -x c /dev/null \