Fix signature of SBase_getNumPlugins in prototype

This was different from signature in implementation, leading to the
implementation not being C-exported correctly, and having C++ name mangling:

```console
% nm libsbml.so | grep SBase_getNumPlugins
00000000008f5240 T _Z19SBase_getNumPluginsPK5SBase
```

src/sbml/SBase.h

@@ -5074,7 +5074,7 @@ SBase_hasValidLevelVersionNamespaceCombination(SBase_t *sb);
  */
 LIBSBML_EXTERN
 int
-SBase_getNumPlugins(SBase_t *sb);
+SBase_getNumPlugins(const SBase_t *sb);
 
 /**
  * Returns a plug-in structure (extension interface) for an SBML Level 3