[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