i965: Fix union usage for G++ <= 4.6.

This patch fixes this build error with G++ <= 4.6.

  CXX    test_vf_float_conversions.o
test_vf_float_conversions.cpp: In function ‘unsigned int f2u(float)’:
test_vf_float_conversions.cpp:63:20: error: expected primary-expression before ‘.’ token

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86939
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/test_vf_float_conversions.cpp

@@ -60,7 +60,8 @@ union fu {
 static unsigned
 f2u(float f)
 {
-   union fu fu = { .f = f };
+   union fu fu;
+   fu.f = f;
    return fu.u;
 }