MipsTargetStreamer.h - remove unnecessary MipsABIFlagsSection forward declaration. NFC.

We need to include MipsABIFlagsSection.h already

Author: RKSimon

llvm/lib/Target/Mips/MipsTargetStreamer.h

@@ -19,8 +19,6 @@
 
 namespace llvm {
 
-struct MipsABIFlagsSection;
-
 class MipsTargetStreamer : public MCTargetStreamer {
 public:
   MipsTargetStreamer(MCStreamer &S);