[contrib][linux] Add zstd_common module

The zstd_common module was added upstream in commit
https://github.com/torvalds/linux/commit/637a642f5ca5e850186bb64ac75ebb0f124b458d.

But the kernel specific code was inlined into the library. This commit
switches it to use the out of line method that we use for the other
modules.

contrib/linux-kernel/test/include/linux/module.h

@@ -12,6 +12,8 @@
 
 #define EXPORT_SYMBOL(symbol)                                                  \
   void* __##symbol = symbol
+#define EXPORT_SYMBOL_GPL(symbol)                                              \
+  void* __##symbol = symbol
 #define MODULE_LICENSE(license)
 #define MODULE_DESCRIPTION(description)