mesa: Namespace qualify fma to override ambiguity with fma from math.h

MSVC 2013 version of math.h includes an fma() function.

Cc: "10.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/glsl/builtin_functions.cpp

@@ -3936,7 +3936,7 @@ builtin_builder::_fma(const glsl_type *type)
    ir_variable *c = in_var(type, "c");
    MAKE_SIG(type, gpu_shader5, 3, a, b, c);
 
-   body.emit(ret(fma(a, b, c)));
+   body.emit(ret(ir_builder::fma(a, b, c)));
 
    return sig;
 }