util: fix u_print.cpp build on OpenBSD

move include so va_list will be picked up via stdarg.h

In file included from ../src/util/u_printf.cpp:24:
../src/util/u_printf.h:43:41: error: unknown type name 'va_list'; did you mean '__va_list'?
size_t u_printf_length(const char *fmt, va_list untouched_args);
                                        ^~~~~~~
                                        __va_list
/usr/include/machine/_types.h:126:27: note: '__va_list' declared here
typedef __builtin_va_list       __va_list;
                                ^

and add includes to u_printf.h as suggested by Ilia Mirkin
stdarg.h for va_list and stddef.h for size_t

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13448>

src/util/u_printf.cpp

@@ -21,9 +21,9 @@
 //
 // Extract from Serge's printf clover code by airlied.
 
-#include "u_printf.h"
 #include <assert.h>
 #include <stdarg.h>
+#include "u_printf.h"
 #include "util/macros.h"
 
 /* Some versions of MinGW are missing _vscprintf's declaration, although they