x86, msr: fix msr-reg.S compilation with gas 2.16.1

msr-reg.S used the :req option on a macro argument, which wasn't
supported by gas 2.16.1 (but apparently by some earlier versions of
gas, just to be confusing.)  It isn't necessary, so just remove it.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Borislav Petkov <petkovbb@googlemail.com>

arch/x86/lib/msr-reg.S

@@ -11,7 +11,7 @@
  * reg layout: u32 gprs[eax, ecx, edx, ebx, esp, ebp, esi, edi]
  *
  */
-.macro op_safe_regs op:req
+.macro op_safe_regs op
 ENTRY(native_\op\()_safe_regs)
 	CFI_STARTPROC
 	pushq_cfi %rbx