qbluetoothdevicediscoveryagent_p.cpp not needed on iOS, since
for iOS we already have iOS-specific version defined completely in the
corresponding .mm file. This is quite obvious ODR violation, but not
so obvious for compiler/linker apparently.

Change-Id: I7b7d0c9596cccea307ec7acbafbb6cd31b41d885
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>