Vulkan: Fix GLSL compilation errors.

Recent changes in our GLSL libraries didn't compile on Vulkan. This
change reverts a compile directive that was removed, but required
in order to compile using the Vulkan backend.

source/blender/gpu/shaders/common/gpu_shader_math_matrix_lib.glsl

@@ -469,6 +469,7 @@ mat4x4 invert(mat4x4 mat, out bool r_success)
   return r_success ? inverse(mat) : mat4x4(0.0);
 }
 
+#  ifdef GPU_METAL
 vec2 normalize(vec2 a)
 {
   return a * inversesqrt(length_squared(a));
@@ -481,6 +482,7 @@ vec4 normalize(vec4 a)
 {
   return a * inversesqrt(length_squared(a));
 }
+#  endif
 
 mat2x2 normalize(mat2x2 mat)
 {