scudo: Fix the build of wrappers_c_test.cpp on Android.

The Android headers don't provide a declaration of valloc or pvalloc, so we
need to declare them ourselves.

Differential Revision: https://reviews.llvm.org/D71077

compiler-rt/lib/scudo/standalone/tests/wrappers_c_test.cpp

@@ -20,6 +20,8 @@ void malloc_disable(void);
 int malloc_iterate(uintptr_t base, size_t size,
                    void (*callback)(uintptr_t base, size_t size, void *arg),
                    void *arg);
+void *valloc(size_t size);
+void *pvalloc(size_t size);
 }
 
 // Note that every C allocation function in the test binary will be fulfilled