ppc64: compile fix - define execve in misc.S

This used to be inline in include/asm-ppc64/unistd.h, but isn't
inline in the merged include/asm-powerpc/unistd.h, so we need a
definition here.
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/ppc64/kernel/misc.S

@@ -740,6 +740,13 @@ _GLOBAL(giveup_altivec)
 _GLOBAL(__setup_cpu_power3)
 	blr
 
+_GLOBAL(execve)
+	li	r0,__NR_execve
+	sc
+	bnslr
+	neg	r3,r3
+	blr
+
 /* kexec_wait(phys_cpu)
  *
  * wait for the flag to change, indicating this kernel is going away but