Don't define MODULE_NAME as a string literal

The `MODULE_NAME` macro is used in contexts where a string literal is
not valid, but the fallback value set in `src/common.h` defines it as
such; this differs from how it is defined in `setup.py`. Define
`MODULE_NAME` in `src/common.h` as it is defined in `setup.py`.

Fixes #267.

Author: chrisnovakovic

src/common.h

@@ -13,7 +13,7 @@
 #include "debug.h"
 
 #ifndef MODULE_NAME
-#define MODULE_NAME "xmlsec"
+#define MODULE_NAME xmlsec
 #endif
 
 #define JOIN(X,Y) DO_JOIN1(X,Y)