Add kInstErrorMax to instrument.hpp (#4315)

To allow for extending GPU-AV error codes outside of spirv-tools.
diff --git a/include/spirv-tools/instrument.hpp b/include/spirv-tools/instrument.hpp
index 05a0472..a19491f 100644
--- a/include/spirv-tools/instrument.hpp
+++ b/include/spirv-tools/instrument.hpp
@@ -178,6 +178,7 @@
 static const int kInstErrorBuffOOBStorage = 5;
 static const int kInstErrorBuffOOBUniformTexel = 6;
 static const int kInstErrorBuffOOBStorageTexel = 7;
+static const int kInstErrorMax = kInstErrorBuffOOBStorageTexel;
 
 // Direct Input Buffer Offsets
 //