MIPS: process: include exec.h header in process.c

arch/mips/kernel/process.c:696:15: error: no previous prototype for 'arch_align_stack' [-Werror=missing-prototypes]
Signed-off-by: Pujin Shi <shipujin.t@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/kernel/process.c

@@ -52,6 +52,7 @@
 #include <asm/inst.h>
 #include <asm/stacktrace.h>
 #include <asm/irq_regs.h>
+#include <asm/exec.h>
 
 #ifdef CONFIG_HOTPLUG_CPU
 void arch_cpu_idle_dead(void)