Add const.

llvm-svn: 205013
diff --git a/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp b/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp
index e11a5f3..190aa14 100644
--- a/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp
+++ b/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp
@@ -254,7 +254,7 @@
                                        const MCAsmLayout &Layout,
                                        const MCFixup &Fixup,
                                        const MCFragment *DF,
-                                       MCValue &Target,
+                                       const MCValue &Target,
                                        uint64_t &Value,
                                        bool &IsResolved) {
   // At this point we'll ignore the value returned by adjustFixupValue as