radeonsi: fix build with GCC 4.8

Fixes: 7088b655e8 ("radeonsi: constify a bunch of the perfcounter structs.")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100937
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/drivers/radeonsi/si_perfcounter.c

@@ -302,7 +302,7 @@ static const struct si_pc_block_base cik_TCP = {
 	.layout = SI_PC_MULTI_ALTERNATE,
 };
 
-static const struct si_pc_block_base cik_VGT = (const struct si_pc_block_base) {
+static const struct si_pc_block_base cik_VGT = {
 	.name = "VGT",
 	.num_counters = 4,
 	.flags = R600_PC_BLOCK_SE,