Add required libqt5svg5 library into developer documentation (#4640)

* add required libqt5svg5 library into developer doc

Run `sudo apt install libqt5svg5-dev` before compiling libopenshot

* Add required libxcb-xfixes0-dev pkg 4 make install

Author: bieli

doc/developers.rst

@@ -101,6 +101,8 @@ unofficial Ubuntu repository, which has our software packages available to downl
                         python3-dev \
                         protobuf-compiler \
                         qtbase5-dev \
+                        libqt5svg5-dev \
+                        libxcb-xfixes0-dev \
                         qtmultimedia5-dev \
                         swig