gallivm: Include stddef.h before the LLVM C++ headers.

Necessary with build against LLVM 2.6, with recent gcc, as LLVM headers
depend on ptrdiff_t but don't properly include stddef.h

src/gallium/auxiliary/gallivm/lp_bld_debug.cpp

@@ -25,6 +25,8 @@
  *
  **************************************************************************/
 
+#include <stddef.h>
+
 #include <llvm-c/Core.h>
 #include <llvm/Target/TargetMachine.h>
 #include <llvm/Target/TargetInstrInfo.h>

src/gallium/auxiliary/gallivm/lp_bld_misc.cpp

@@ -34,6 +34,8 @@
 #define __STDC_CONSTANT_MACROS
 #endif
 
+#include <stddef.h>
+
 #include <llvm-c/Core.h>
 #include <llvm-c/ExecutionEngine.h>
 #include <llvm/Target/TargetOptions.h>