[flang][nfc] Add missing REQUIRES directive in a test

Failing bot:
  * https://lab.llvm.org/buildbot/#/builders/21/builds/88731

Failing test was introduced in:
  * #74598

Sending without a review as the fix is straightforward and I want to
prioritize fixing the broken bot and unblocking everyone who's been
affected.

Author: banach-space

flang/test/Driver/func-attr.f90

@@ -1,5 +1,7 @@
 ! Test that -mframe-pointer can accept only specific values and when given an invalid value, check it raises an error.
 
+! REQUIRES: aarch64-registered-target
+
 ! RUN: %flang_fc1 -triple aarch64-none-none -mframe-pointer=none -emit-llvm -o - %s 2>&1| FileCheck %s --check-prefix=CHECK-NONEFP
 ! RUN: %flang_fc1 -triple aarch64-none-none -mframe-pointer=non-leaf -emit-llvm -o - %s 2>&1| FileCheck %s --check-prefix=CHECK-NONLEAFFP
 ! RUN: %flang_fc1 -triple aarch64-none-none -mframe-pointer=all -emit-llvm -o - %s 2>&1| FileCheck %s --check-prefix=CHECK-ALLFP