asm-generic/export.h: make __ksymtab_* local symbols

For EXPORT_SYMBOL from C files, <linux/export.h> defines __ksymtab_*
as local symbols.

For EXPORT_SYMBOL from assembly, in contrast, <asm-generic/export.h>
produces globally-visible __ksymtab_* symbols due to this .globl
directive.

I do not know why this must be global. It still works without this.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>