moonray_gui_v2 needs to link libdl explicitly for imGui OpenGL backend

[jlanz]

Problem

When building moonray_gui_v2 against newer toolchains (as used in the ASWF CI
images), the linker emits:

undefined reference to `dlclose@@GLIBC_2.2.5'
undefined reference to `dlopen@@GLIBC_2.2.5'
undefined reference to `dlerror@@GLIBC_2.2.5'

This is a link-time error in moonray_gui_v2

Root Cause

moonray_gui_v2 uses the dynamic linker API (dlopen/dlclose/dlerror) from
libdl. The ASWF CI images for 2023–2025 all use GCC 11.2 (per VFX Reference
Platform), but the linker flag --as-needed (the default on modern Linux
distributions) causes libdl to be dropped when it is not listed as a direct
dependency. Previously libdl was pulled in transitively by another linked
library; with the libraries present in these images it is not, so it must be
listed explicitly.