st/clover: Allow double precision operations

Pass "cl_khr_fp64" preprocessor definition to clang

Signed-off-by: Niels Ole Salscheider <niels_ole@salscheider-online.de>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>

src/gallium/state_trackers/clover/llvm/invocation.cpp

@@ -175,6 +175,7 @@ namespace {
 
       // clc.h requires that this macro be defined:
       c.getPreprocessorOpts().addMacroDef("cl_clang_storage_class_specifiers");
+      c.getPreprocessorOpts().addMacroDef("cl_khr_fp64");
 
       c.getLangOpts().NoBuiltin = true;
       c.getTargetOpts().Triple = triple;