meta: Move declaration before code.

Fixes MSVC build.

meta.c(2411) : error C2143: syntax error : missing ';' before 'type'
meta.c(2411) : error C2143: syntax error : missing ')' before 'type'
meta.c(2411) : error C2065: 'layer' : undeclared identifier
meta.c(2411) : error C2059: syntax error : ')'
meta.c(2411) : error C2143: syntax error : missing ';' before '{'
meta.c(2413) : error C2065: 'layer' : undeclared identifier
meta.c(2415) : error C2065: 'layer' : undeclared identifier

Signed-off-by: Vinson Lee <vlee@freedesktop.org>

src/mesa/drivers/common/meta.c

@@ -2408,7 +2408,8 @@ _mesa_meta_glsl_Clear(struct gl_context *ctx, GLbitfield buffers)
 
    /* draw quad(s) */
    if (fb->NumLayers > 0) {
-      for (unsigned layer = 0; layer < fb->NumLayers; layer++) {
+      unsigned layer;
+      for (layer = 0; layer < fb->NumLayers; layer++) {
          if (fb->_IntegerColor)
             _mesa_Uniform1i(clear->IntegerLayerLocation, layer);
          else