selftests/bpf: Correct typo in __clang_major__ macro

Make sure that CAN_USE_BPF_ST test (compute_live_registers/store) is
enabled when __clang_major__ >= 18.

Fixes: 2ea8f6a ("selftests/bpf: test cases for compute_live_registers()")
Signed-off-by: Peilin Ye <yepeilin@google.com>
Link: https://lore.kernel.org/r/20250425213712.1542077-1-yepeilin@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: peilin-ye

tools/testing/selftests/bpf/progs/bpf_misc.h

@@ -221,7 +221,7 @@
 #define CAN_USE_GOTOL
 #endif
 
-#if _clang_major__ >= 18
+#if __clang_major__ >= 18
 #define CAN_USE_BPF_ST
 #endif