cmake: fix building cgame Windows dll with MinGW

[illwieckz]

On my end this fixes the build of Windows game dll with MinGW on Linux (Ubuntu 23.10), otherwise I get that:

[100%] Linking CXX shared module cgame-native-dll.dll
/usr/bin/x86_64-w64-mingw32-ld: windows-amd64-mingw_9/lib/libfreetype.a(ftgzip.o):(.text+0x17c): undefined reference to `inflateEnd'
/usr/bin/x86_64-w64-mingw32-ld: windows-amd64-mingw_9/lib/libfreetype.a(ftgzip.o):(.text+0x268): undefined reference to `inflate'
/usr/bin/x86_64-w64-mingw32-ld: windows-amd64-mingw_9/lib/libfreetype.a(ftgzip.o):(.text+0x4e8): undefined reference to `inflateReset'
/usr/bin/x86_64-w64-mingw32-ld: windows-amd64-mingw_9/lib/libfreetype.a(ftgzip.o):(.text+0x689): undefined reference to `inflateInit2_'
/usr/bin/x86_64-w64-mingw32-ld: windows-amd64-mingw_9/lib/libfreetype.a(ftgzip.o):(.text+0x7f3): undefined reference to `inflateEnd'
/usr/bin/x86_64-w64-mingw32-ld: windows-amd64-mingw_9/lib/libfreetype.a(ftgzip.o):(.text+0x920): undefined reference to `inflateInit2_'
/usr/bin/x86_64-w64-mingw32-ld: windows-amd64-mingw_9/lib/libfreetype.a(ftgzip.o):(.text+0x933): undefined reference to `inflate'
/usr/bin/x86_64-w64-mingw32-ld: windows-amd64-mingw_9/lib/libfreetype.a(ftgzip.o):(.text+0x942): undefined reference to `inflateEnd'
/usr/bin/x86_64-w64-mingw32-ld: windows-amd64-mingw_9/lib/libfreetype.a(ftgzip.o):(.text+0x99d): undefined reference to `inflateEnd'
collect2: error: ld returned 1 exit status

[slipher]

LGTM