[RPM] Add an explicit dependency for GDAL

[daniviga]

Description

Trying to reproject a raster layer using QGIS from RPM on Fedora I got

Loading resulting layers
The following layers were not correctly generated.<ul><li>/tmp/processing_22a1c797a97348499be837e8f9cd9df8/94bfcb797bf24f8b9bc2c85b8e2d409d/OUTPUT.tif</li></ul>You can check the 'Log Messages Panel' in QGIS main window to find more information about the execution of the algorithm.

Because GDAL was not installed as dependency of QGIS by default. I'm modifying the RPM to require GDAL:

Results:
{'OUTPUT': <QgsProcessingOutputLayerDefinition {'sink':/tmp/processing_22a1c797a97348499be837e8f9cd9df8/6ae649116d784d47bfc1cb44068738b6/OUTPUT.tif, 'createOptions': {'fileEncoding': 'System'}}>}

Loading resulting layers
Algorithm 'Warp (reproject)' finished

Checklist

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• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
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