Make it possible to build pgmodeler out of tree

[pinaraf]

Out of tree builds were failing because the uic files are included while being generated in the build directory.
This fixes it by including the proper folder.

[Maxzor]

Hello, I am learning qt build system,
shouldn't we use :
UI_DIR - Specifies the directory where all intermediate files from uic should be placed.
instead of
OUT_PWD - Specifies the full path leading to the directory where qmake places the generated Makefile. Note: Do not attempt to overwrite the value of this variable.
(which is written by PREFIX AFAIK)
https://doc.qt.io/qt-5/qmake-variable-reference.html