package/ibrcommon: fix static build with openssl

gf_mul is already defined in libcrypto (openssl) so rename it into
ibrdtn_gf_mul to fix the following build failure in ibrdtnd package:

/home/buildroot/autobuild/instance-3/output/host/bin/../arm-buildroot-uclinux-uclibcgnueabi/sysroot/usr/lib/libcrypto.a(f_impl.o): In function `gf_mul':
f_impl.c:(.text+0x0): multiple definition of `gf_mul'
/home/buildroot/autobuild/instance-3/output/host/arm-buildroot-uclinux-uclibcgnueabi/sysroot/usr/lib/libibrcommon.a(gf128mul.o):gf128mul.cpp:(.text+0x30): first defined here
collect2: error: ld returned 1 exit status
Makefile:560: recipe for target 'dtnd' failed

Fixes:
 - http://autobuild.buildroot.org/results/1d3b4b6cf043a3e185ce758b617a0a18c3d36cdb



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>