[202012][sonic-platform-common] Track 202012 branch of submodule (#7326)

Recently created a 202012 branch of sonic-platform common. 

Update .gitmodules to track that branch of the submodule in the 202012 branch of sonic-buildimage.

Author: jleveque

.gitmodules

@@ -41,6 +41,7 @@
 [submodule "src/sonic-platform-common"]
 	path = src/sonic-platform-common
 	url = https://github.com/Azure/sonic-platform-common
+	branch = 202012
 [submodule "src/sonic-platform-daemons"]
 	path = src/sonic-platform-daemons
 	url = https://github.com/Azure/sonic-platform-daemons