Address some compile warnings from Clang 13

The latest emscripten uses Clang 13, which points out some new warnings
about implicit copy constructors, copy assignment operators etc.

This addresses some that I found until llvm crashed and would not
compile further (https://bugs.llvm.org/show_bug.cgi?id=50408)

Change-Id: Ia4d3f0b308facb18bf2da50261326244a7848644
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/413056
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
diff --git a/src/gpu/GrProgramDesc.h b/src/gpu/GrProgramDesc.h
index 96dec23..cac31a9 100644
--- a/src/gpu/GrProgramDesc.h
+++ b/src/gpu/GrProgramDesc.h
@@ -106,6 +106,7 @@
 class GrProgramDesc {
 public:
     GrProgramDesc(const GrProgramDesc& other) = default;
+    GrProgramDesc& operator=(const GrProgramDesc &other) = default;
 
     bool isValid() const { return !fKey.empty(); }
     void reset() { *this = GrProgramDesc{}; }