Fix Clang warning about comparison of different signs

Commit 4d35f66a

 changed from -1 to ~0 because
ICC complained, but Clang still sees ~0 as int. So force to unsigned.

qgstappsrc.cpp:144:31: warning: comparison of integers of different signs: 'unsigned int' and 'int' [-Wsign-compare]

Change-Id: I6b0fdf8dc5df157d8e184f22dba44660e60ca3b2
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>