Fix extension when looking for mlir_c_runner library (#649)

**Context:** When merging callbacks, something that was an oversight was
the extension of shared libraries in different operating systems. In
Darwin, the extension is `.dylib`. In unix, it is `.so`.

**Description of the Change:** This commit selects the correct extension
based on the operating system that the code is being run.

**Benefits:** No errors on tests.

doc/changelog.md

@@ -7,6 +7,7 @@
   [(#596)](https://github.com/PennyLaneAI/catalyst/pull/596)
   [(#610)](https://github.com/PennyLaneAI/catalyst/pull/610)
   [(#650)](https://github.com/PennyLaneAI/catalyst/pull/650)
+  [(#649)](https://github.com/PennyLaneAI/catalyst/pull/649)
 
   Catalyst now supports callbacks with parameters and return values.
   The following is now possible:

runtime/lib/registry/Registry.cpp

@@ -72,6 +72,19 @@ class LibraryManager {
     }
 };
 
+inline const char *ext()
+{
+#ifdef __APPLE__
+    return ".dylib";
+#elif __linux__
+    return ".so";
+#else
+#error "Only apple and linux are currently supported";
+#endif
+}
+
+std::string library_name(std::string name) { return name + ext(); }
+
 void convertResult(py::handle tuple)
 {
     py::object unrankedMemrefPtrSizeTuple = tuple.attr("__getitem__")(0);
@@ -91,7 +104,7 @@ void convertResult(py::handle tuple)
     UnrankedMemrefType *src = (UnrankedMemrefType *)unranked_memref_ptr;
     UnrankedMemrefType destMemref = {src->rank, destAsPtr};
 
-    std::string libpath = libmlirpath + "/libmlir_c_runner_utils.so";
+    std::string libpath = libmlirpath + library_name("/libmlir_c_runner_utils");
     LibraryManager memrefCopy(libpath);
     memrefCopy(e_size, src, &destMemref);
 }