[OpenMP][Obvious] Fix function prototype when used in C mode

Summary:
The `llvm_omp_target_dynamic_shared_alloc` prototype in `omp.h`
accidentally left the void argument unspecified. This created unintended
code when called from the C language, causing some `nvlink` failures in
certain scenarios.

Author: jhuber6

openmp/runtime/src/include/omp.h.var

@@ -502,7 +502,7 @@
     extern int __KAI_KMPC_CONVENTION omp_in_explicit_task(void);
 
     /* LLVM Extensions */
-    extern void *llvm_omp_target_dynamic_shared_alloc();
+    extern void *llvm_omp_target_dynamic_shared_alloc(void);
 
 #   undef __KAI_KMPC_CONVENTION
 #   undef __KMP_IMP