[XTENSA] Remove multi-exported symbols from xtensa_ksyms.c

This patch removes the following symbols from
arch/xtensa/kernel/xtensa_ksyms.c that were already exported:
memcmp, memchr, strcat, strchr, strlen, stncat, strnlen, strrchr,
strstr, enable_irq, and disable_irq

Signed-off-by: Chris Zankel <chris@zankel.net>