Include LLVMSPIRVLib.h in SPIRVError.cpp (#2377)

The prototype for `getErrorMessage()` is declared in `LLVMSPIRVLib.h`,
but `SPIRVError.cpp` defining that function did not include
`LLVMSPIRVLib.h`.  This can be problematic for builds that use
`-fvisibility=hidden`.

Fixes KhronosGroup/SPIRV-LLVM-Translator#2376

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@81f78d24db36622

Author: svenvh

llvm-spirv/lib/SPIRV/libSPIRV/SPIRVError.cpp

@@ -36,6 +36,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "LLVMSPIRVLib.h"
+
 #include "SPIRVError.h"
 #include <string>