Fix compiling imgui with boost libraries (#1755)

Addind only the include dirs is wrong. The cmake boost_headeronly
library contains more settings.

Without adding the cmake boost_headeronly library, the compilation of
imgui is missing the defines for the boost library (f.e.
BOOST_LOG_DYN_LINK) and it will result in linker errors.

Co-authored-by: SoftFever <softfeverever@gmail.com>

src/imgui/CMakeLists.txt

@@ -16,5 +16,5 @@ add_library(imgui STATIC
 )
 
 if(Boost_FOUND)
-    include_directories(${Boost_INCLUDE_DIRS})
-endif()
+    target_link_libraries(imgui PRIVATE boost_headeronly)
+endif()