add object_script to Qt.gitignore

For template rules (modern style) in Qt project file,
this file is autogenerated by qmake.

Example old style (pro file):
SOURCES += src/1.cpp \
           src/2.cpp

Example new style, uses template rules (pro file):
SOURCES += src/*.cpp

qmake auto generate files with list of objects
object_script.*.Debug
object_script.*.Release