modpost: remove left-over cross_compile declaration

This is a remnant of commit 6543becf ("mod/file2alias: make
modalias generation safe for cross compiling").
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

scripts/mod/modpost.h

@@ -174,7 +174,6 @@ static inline unsigned int get_secindex(const struct elf_info *info,
 }
 
 /* file2alias.c */
-extern unsigned int cross_build;
 void handle_moddevtable(struct module *mod, struct elf_info *info,
 			Elf_Sym *sym, const char *symname);
 void add_moddevtable(struct buffer *buf, struct module *mod);