spirv: Generate missing SampleMaskOverrideCoverageNV capability op

The SampleMaskOverrideCoverageNV capability should be declared when
gl_SampleMask is used.
diff --git a/SPIRV/GlslangToSpv.cpp b/SPIRV/GlslangToSpv.cpp
index 73a8f6a..fabaee9 100644
--- a/SPIRV/GlslangToSpv.cpp
+++ b/SPIRV/GlslangToSpv.cpp
@@ -7738,6 +7738,7 @@
               decoration = (spv::Decoration)spv::DecorationMax;
         builder.addDecoration(id, decoration);
         if (decoration != spv::DecorationMax) {
+            builder.addCapability(spv::CapabilitySampleMaskOverrideCoverageNV);
             builder.addExtension(spv::E_SPV_NV_sample_mask_override_coverage);
         }
     }