arm: add elf.h to arch/arm/kernel/ptrace.c

It was implicitly getting it via an implicit presence of module.h
but when we clean that up, we'll get a bunch of lines like this:

arch/arm/kernel/ptrace.c:764: error: 'NT_PRSTATUS' undeclared here (not in a function)
arch/arm/kernel/ptrace.c:765: error: 'ELF_NGREG' undeclared here (not in a function)
arch/arm/kernel/ptrace.c:776: error: 'NT_PRFPREG' undeclared here (not in a function)
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

arch/arm/kernel/ptrace.c

@@ -12,6 +12,7 @@
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/mm.h>
+#include <linux/elf.h>
 #include <linux/smp.h>
 #include <linux/ptrace.h>
 #include <linux/user.h>