Add one more definition for symbols in prctl unit test.

one more follow up to: https://reviews.llvm.org/D122851

Differential Revision: https://reviews.llvm.org/D123404

compiler-rt/test/sanitizer_common/TestCases/Linux/prctl.cpp

@@ -13,6 +13,10 @@
 #  define PR_SCHED_CORE_CREATE 1
 #endif
 
+#ifndef PR_SCHED_CORE_GET
+#  define PR_SCHED_CORE_GET 0
+#endif
+
 int main() {
 
   int res;