ice: xsk: fix GCC version checking against pragma unroll presence

Pragma unroll was introduced around GCC 8, whereas current xsk code in
ice that prepares loop_unrolled_for macro that is based on mentioned
pragma, compares GCC version against 4, which is wrong and Stephen
found this out by compiling kernel with GCC 5.4 [0].

Fix this mistake and check if GCC version is >= 8.

[0]: https://lore.kernel.org/netdev/20220307213659.47658125@canb.auug.org.au/

Fixes: 126cdfe ("ice: xsk: Improve AF_XDP ZC Tx and use batching API")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20220307231353.56638-1-maciej.fijalkowski@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/intel/ice/ice_xsk.h

@@ -10,7 +10,7 @@
 
 #ifdef __clang__
 #define loop_unrolled_for _Pragma("clang loop unroll_count(8)") for
-#elif __GNUC__ >= 4
+#elif __GNUC__ >= 8
 #define loop_unrolled_for _Pragma("GCC unroll 8") for
 #else
 #define loop_unrolled_for for