[llvm-c] Make LLVMGetStringErrorTypeId a proper prototype

In C a function declaration with an empty argument list isn't a real
prototype, it will allow calling the function with any number of
arguments. It will also cause warnings when used in C code compiled with
'-Wstrict-prototypes'

Reviewed By: whitequark
Differential Revision: https://reviews.llvm.org/D61568

llvm-svn: 360012

llvm/include/llvm-c/Error.h

@@ -60,7 +60,7 @@ void LLVMDisposeErrorMessage(char *ErrMsg);
 /**
  * Returns the type id for llvm StringError.
  */
-LLVMErrorTypeId LLVMGetStringErrorTypeId();
+LLVMErrorTypeId LLVMGetStringErrorTypeId(void);
 
 #ifdef __cplusplus
 }