IMGUI: Link against -ldl now

[emminizer]

Something in the recent IMGUI update broke the link command on Linux for my EL7 and EL8 systems for examples. I got errors like the following:

[ 79%] Linking CXX executable ../../../bin/osgearth_imgui
/opt/rh/gcc-toolset-12/root/usr/libexec/gcc/x86_64-redhat-linux/12/ld: CMakeFiles/application_osgearth_imgui.dir/__/__/third_party/imgui/backends/imgui_impl_opengl3.cpp.o: undefined reference to symbol 'dlclose@@GLIBC_2.2.5'
/opt/rh/gcc-toolset-12/root/usr/libexec/gcc/x86_64-redhat-linux/12/ld: /lib64/libdl.so.2: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make[2]: *** [src/applications/osgearth_imgui/CMakeFiles/application_osgearth_imgui.dir/build.make:222: bin/osgearth_imgui] Error 1

This patch adds in -ldl wherever IMGUI is linked in, using the existing macro, using the automatically defined CMAKE_DL_LIBS. I tested this on RHEL7 gcc 11.2.1 and on MSVC 2022 x64.