util: add log2f for Android

It is needed for nv50's new shader backend.  With this change, both u_math.h
and imports.h in core mesa define the same function.  I have to #undef log2f
here to avoid the conflict.  Not sure if there is a better way to deal with
the situation.

Acked-by: José Fonseca <jfonseca@vmware.com>

src/gallium/auxiliary/util/u_math.h

@@ -207,6 +207,17 @@ double log2(double d)
    return log(d) * (1.0 / M_LN2);
 }
 
+/* workaround a conflict with main/imports.h */
+#ifdef log2f
+#undef log2f
+#endif
+
+static INLINE
+float log2f(float f)
+{
+   return logf(f) * (float) (1.0 / M_LN2);
+}
+
 #endif