nv50/ra: `isinf()` is in namespace `std` since C++11.

This fixes a compile error while building Nouveau with C++11 enabled (and
glibc >= 2.23). This happens if SWR is enabled, as it forces C++11.
Signed-off-by: Pierre Moreau <pierre.morrow@free.fr>
Signed-off-by: Jose Fonseca <jfonseca@vmware.com>

https://bugs.freedesktop.org/show_bug.cgi?id=94907

src/gallium/drivers/nouveau/codegen/nv50_ir_ra.cpp

@@ -1327,7 +1327,11 @@ GCRA::simplify()
                bestScore = score;
             }
          }
+#if __cplusplus >= 201103L
+         if (std::isinf(bestScore)) {
+#else
          if (isinf(bestScore)) {
+#endif
             ERROR("no viable spill candidates left\n");
             break;
          }