fixing warnings

I got these warnings with such flags CFLAGS = -pedantic -Wall -Wextra:

    clang  -pedantic -Wall -Wextra  main.c -c -o main.o
    main.c:100:29: warning: variable length array folded to constant array as an
          extension [-Wpedantic]
      CGDirectDisplayID display[kMaxDisplays];
                                ^~~~~~~~~~~~
    main.c:124:38: warning: 'CGDisplayCurrentMode' is deprecated: first deprecated
          in OS X 10.6 [-Wdeprecated-declarations]
        CFDictionaryRef   originalMode = CGDisplayCurrentMode(dspy);
                                         ^
    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/System/Library/Frameworks/CoreGraphics.framework/Headers/CGDirectDisplay.h:455:27: note: 
          'CGDisplayCurrentMode' declared here
    CG_EXTERN CFDictionaryRef CGDisplayCurrentMode(CGDirectDisplayID display)
                              ^
    main.c:143:28: warning: 'CGDisplayIOServicePort' is deprecated: first deprecated
          in OS X 10.9 [-Wdeprecated-declarations]
        io_service_t service = CGDisplayIOServicePort(dspy);
                               ^
    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/System/Library/Frameworks/CoreGraphics.framework/Headers/CGDisplayConfiguration.h:355:24: note: 
          'CGDisplayIOServicePort' declared here
    CG_EXTERN io_service_t CGDisplayIOServicePort(CGDirectDisplayID display)
                           ^
    main.c:180:2: warning: no newline at end of file [-Wnewline-eof]
    }
     ^
    4 warnings generated.