Use the properly-capitalized C-ABI MKL threading interface

It appears that the all-lowercase threading symbols are actually all
FORTRAN ABI, as can be seen through `dlsym()`:

```
julia> dlsym(libmkl_rt, "mkl_set_num_threads")
Ptr{Nothing} @0x00007fa64f24e3b0

julia> dlsym(libmkl_rt, "mkl_set_num_threads_")
Ptr{Nothing} @0x00007fa64f24e3b0
```

The C-ABI versions are not the non-underscore-suffixed ones, but the
capitalized ones, e.g. `MKL_Set_Num_Threads`.  The more you know.

Author: staticfloat

src/threading.c

@@ -10,14 +10,14 @@
  */
 static char * getter_names[MAX_THREADING_NAMES] = {
     "openblas_get_num_threads",
-    "mkl_get_max_threads",
+    "MKL_Get_Max_Threads",
     "bli_thread_set_num_threads",
     NULL
 };
 
 static char * setter_names[MAX_THREADING_NAMES] = {
     "openblas_set_num_threads",
-    "mkl_set_num_threads",
+    "MKL_Set_Num_Threads",
     "bli_thread_get_num_threads",
     NULL
 };