mips: fix implicit use of asm/elf.h in kernel/cpu-probe.c

We are relying on asm/elf.h being present implicitly.  Once we clean
up the root cause of that, we'll see this, so fix it in advance.

arch/mips/kernel/cpu-probe.c: In function 'set_elf_platform':
arch/mips/kernel/cpu-probe.c:298: error: '__elf_platform' undeclared (first use in this function)

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>