Fix compiler error on uncasted objc_msgSend calls in Xcode 6.2

[rlindner]

This fixes a compiler error in Xcode 6.2 by casting objc_msgSend calls, as stated in the comments of objc/message.h

/* Basic Messaging Primitives
 *
 * On some architectures, use objc_msgSend_stret for some struct return types.
 * On some architectures, use objc_msgSend_fpret for some float return types.
 * On some architectures, use objc_msgSend_fp2ret for some float return types.
 *
 * These functions must be cast to an appropriate function pointer type 
 * before being called. 
 */
#if !OBJC_OLD_DISPATCH_PROTOTYPES
OBJC_EXPORT void objc_msgSend(void /* id self, SEL op, ... */ )
    __OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);
OBJC_EXPORT void objc_msgSendSuper(void /* struct objc_super *super, SEL op, ... */ )
    __OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);
#else