Use -fPIC when compiling using dpkg-buildpackage.

The makefile of this plugin uses the following line, which only sets
CFLAGS when it isn't already set:

CFLAGS  ?= -g -pipe -fPIC

As when called in Debian's build enviroment, CFLAGS is already set,
CFLAGS has to be set before make is called via debian/rules.
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