Disable werror for Dear ImGui

GCC 7.5:
../../subprojects/imgui-1.81/imgui.h:1642:80: warning: assuming signed overflow does not occur when assuming that (X - c) <= X is always true [-Wstrict-overflow]
     inline T&           operator[](int i)                   { IM_ASSERT(i >= 0 && i < Size); return Data[i]; }

meson.build

@@ -218,6 +218,7 @@ util_files = files(
 
 imgui_options = [
   'default_library=static',
+  'werror=false',
   # use 'auto_features=disabled' once available: https://github.com/mesonbuild/meson/issues/5320
   'dx9=disabled',
   'dx10=disabled',