Add -fno-omit-interface-pragmas to SrcLoc, to work around Trac #5252

We have to do this until the earliest bootstrap compiler
has fixed #5252.  It's no big deal; it just means that
SrcLoc.hi has lots of stuff even when compiling GHC without -O.
Lacking this, you get

  (GHC version 7.0.3 for i386-unknown-mingw32):
	reboxProduct: not a product ghc-7.1:SrcLoc.RealSrcLoc{tc r8J}