[NFC][compiler-rt][fuchsia] Add InitShadowBounds declaration to header

Forgot to include this as a part of
a11aea68a4.

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.h

@@ -30,6 +30,8 @@ struct MemoryMappingLayoutData {
   size_t current;  // Current index into the vector.
 };
 
+void InitShadowBounds();
+
 }  // namespace __sanitizer
 
 #endif  // SANITIZER_FUCHSIA