Add function prototype for gunzip() to the boot library code and make
exit() and zalloc() static.

arch/xtensa/boot/lib/zmem.c:8:6: warning: no previous prototype for 'exit' [-Wmissing-prototypes]
    8 | void exit (void)
arch/xtensa/boot/lib/zmem.c:13:7: warning: no previous prototype for 'zalloc' [-Wmissing-prototypes]
   13 | void *zalloc(unsigned size)
arch/xtensa/boot/lib/zmem.c:35:6: warning: no previous prototype for 'gunzip' [-Wmissing-prototypes]
   35 | void gunzip (void *dst, int dstlen, unsigned char *src, int *lenp)

Fixes: 4bedea94 ("xtensa: Architecture support for Tensilica Xtensa Part 2")
Fixes: e7d163f7 ("xtensa: Removed local copy of zlib and fixed O= support")
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>