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.
2 jobs for librem5 in 1 minute and 58 seconds
latest
Status Job ID Name Coverage
  Build
passed #145756
librem5
build-debian-gcc

00:00:59

 
  Package
passed #145757
librem5
build-debian-package

00:00:58