CMake: Fix the include guards in rfbconfig.h

The include guard shouldn't be a cmakedefine - this is not a 0/1
define that we expect cmake to configure, it should always be
defined here.

This avoids warnings like this (when building with Clang, targeting
Windows), due to rfbconfig.h being included multiple times, when
building with CMake:

    libvncserver/build/include/rfb/rfbconfig.h:197:13: warning: redefinition of typedef 'socklen_t' is a C11 feature [-Wtypedef-redefinition]
      197 | typedef int socklen_t;
          |             ^
    libvncserver/include/rfb/rfbproto.h:70:10: note: 'libvncserver/build/include/rfb/rfbconfig.h' included multiple times, additional include site here
       70 | #include <rfb/rfbconfig.h>
          |          ^
    libvncserver/include/rfb/threading.h:25:10: note: 'libvncserver/build/include/rfb/rfbconfig.h' included multiple times, additional include site here
       25 | #include <rfb/rfbconfig.h>
          |          ^