objtool: Only include valid definitions depending on source file type

Header include/linux/objtool.h contains both C and assembly definition that
are visible regardless of the file including them.

Place definition under conditional __ASSEMBLY__.

Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Julien Thierry <jthierry@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>