kbuild: $(CHECK) doesnt need NOSTDINC_FLAGS twice

Currently, $(CHECK) receives NOSTDINC_FLAGS twice:
* first directly in the main Makefile via CHECKFLAGS,
* then indirectly in scripts/Makefile.build via c_flags.

Since once is enough, leave the occurence via c_flags and
remove the one via CHECKFLAGS.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>