Fix RelWithDebug compile issue in gltf\asset.cpp

In indra\newview\gltf\asset.cpp when you compile in Visual Studio for windows with RelWithDebug, there is a compiling error due to #if being used instead #ifdef as the rest of the codebase uses. This is with the latest Visual Studio  17.14.11.

The error given is
Severity	Code	Description	Project	File	Line	Suppression State	Details
Error (active)	E0029	expected an expression	firestorm-bin	G:\minerjr\firestorm\phoenix-firestorm\indra\newview\gltf\asset.cpp	592

Just changed #if's to #ifdef.

Also moved the second #if to #ifdef on line 611 up before the assert as in Release both checks use the attribute_mask which is not defined as its only exists when SHOW_ASSERT exists.

indra/newview/gltf/asset.cpp

@@ -589,7 +589,7 @@ bool Asset::prep()
 
                 for (U32 variant = 0; variant < LLGLSLShader::NUM_GLTF_VARIANTS; ++variant)
                 {
-#if SHOW_ASSERT
+#ifdef SHOW_ASSERT
                     U32 attribute_mask = 0;
 #endif
                     // for each mesh
@@ -606,10 +606,10 @@ bool Asset::prep()
 
                                 vertex_count[variant] += primitive.getVertexCount();
                                 index_count[variant] += primitive.getIndexCount();
-
+#ifdef SHOW_ASSERT
                                 // all primitives of a given variant and material should all have the same attribute mask
                                 llassert(attribute_mask == 0 || primitive.mAttributeMask == attribute_mask);
-#if SHOW_ASSERT
+
                                 attribute_mask |= primitive.mAttributeMask;
 #endif
                             }