Fix annoying unused variable warning on ES.

src/Magnum/Version.h

@@ -132,13 +132,13 @@ inline std::pair<Int, Int> version(Version version) {
 
 Always `true` on @ref MAGNUM_TARGET_GLES "OpenGL ES" and WebGL build.
 */
-constexpr bool isVersionES(Version version) {
 #ifndef MAGNUM_TARGET_GLES
+constexpr bool isVersionES(Version version) {
     return Int(version) & Implementation::VersionESMask;
+}
 #else
-    return true;
+constexpr bool isVersionES(Version) { return true; }
 #endif
-}
 
 /** @debugoperatorenum{Magnum::Version} */
 MAGNUM_EXPORT Debug& operator<<(Debug& debug, Version value);