lib source files are missing the sourfile list

**Describe the bug**
In gdbgui, one great feature is that we can select source file in a source file list. This is very convenience for set breakpoints, etc.

But I found that if my project is depended on a library, the source file of the library is not in the list.

**To Reproduce**

In the CMakeLists.txt below, myproj is depended on library mathcore.
The source file of mathcore can't be found in the source file list , even through we can show its code using 'list' command.

```
cmake_minimum_required(VERSION 3.9 FATAL_ERROR)
set(PORJ_NAME myproj)
project(${PORJ_NAME})

add_executable(${PORJ_NAME}
main.cpp
...
)
target_link_libraries (${PORJ_NAME}
PRIVATE
/proj_math/libmathcore.so
...
)
```

mathcore CMakeLists.txt

```
cmake_minimum_required(VERSION 3.9 FATAL_ERROR)
set(PORJ_NAME mathcore)
project(${PORJ_NAME})

add_library(${PORJ_NAME}
/proj_math/mathcore.cpp
...
)
```

One workaround is to add the file manually to the CMakeLists.txt of myproj.

add_executable(${PORJ_NAME}
main.cpp
/proj_math/mathcore.cpp
...

)

**Expected behavior**
/proj_math/mathcore.cpp should be listed in source file list

**Please complete the following information:**
* OS: ubuntu 16.04
* gdbgui version (0.13.1.1):
* gdb version (8.2):
* browser [chrome, firefox]:

**Additional context**
I am using CMake