basic: remove redundant check (#7320)

The check is redundant as the whole block is only evaluated if
__IGNORE_pkey_mprotect is not defined. Change to #else.

src/basic/missing_syscall.h

@@ -373,7 +373,7 @@ static inline int bpf(int cmd, union bpf_attr *attr, size_t size) {
 #      if _MIPS_SIM == _MIPS_SIM_ABI64
 #        define __NR_pkey_mprotect 5323
 #      endif
-#    elif ! defined(__IGNORE_pkey_mprotect)
+#    else
 #      warning "__NR_pkey_mprotect not defined for your architecture"
 #    endif
 #  endif