port: Don't define dlopen etc. when dlfcn.h is available

[cwabbott0]

Otherwise I get the following build error:

In file included from ../../wine/include/wine/port.h:114,
                 from ../../wine/libs/port/getopt.c:31:
/usr/x86_64-w64-mingw32/sys-root/mingw/include/dlfcn.h:53:20: error: conflicting types for 'dlerror'
   53 | DLFCN_EXPORT char *dlerror(void);
      |                    ^~~~~~~
In file included from ../../wine/libs/port/getopt.c:31:
../../wine/include/wine/port.h:61:27: note: previous definition of 'dlerror' was here
   61 | static inline const char *dlerror(void) { return "No dlopen support on Windows"; }
      |                           ^~~~~~~
make[2]: *** [Makefile:196739: libs/port/getopt.o] Error 1

This seems to be because building the guest wine on aarch64 uses mingw as the "host" compiler and this library is built with the host toolchain.

[AndreRH]

@stefand