arm: allow usage of string functions in linux/string.h

In introducing a trivial "strstarts()" function in linux/string.h, we
hit:

	arch/arm/boot/compressed/misc.o: In function `strstarts':
	misc.c:(.text+0x368): undefined reference to `strlen'
	misc.c:(.text+0x378): undefined reference to `strncmp'

This is because of "CFLAGS_misc.o := -Dstatic=" in the Makefile.
"static inline strstarts(...)" becomes non-inline, and refers to the
other string ops.

The simplest workaround is to include asm/string.h.  This makes sense
anyway, since lib/string.c won't be linked against this so we can't
use those functions anyway.

Compile tested here.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>