package/freeipmi: fix static build with openipmi

Fix the following static build failure with openipmi raised since commit
8f6b346a:

/home/peko/autobuild/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-musleabihf/10.3.0/../../../../arm-buildroot-linux-musleabihf/bin/ld: /home/peko/autobuild/instance-0/output-1/host/arm-buildroot-linux-musleabihf/sysroot/usr/lib/libfreeipmi.a(libfreeipmi_la-ipmi-md5.o): in function `md5_init':
ipmi-md5.c:(.text+0xa7c): multiple definition of `md5_init'; ../lib/.libs/libipmitool.a(md5.o):md5.c:(.text+0xab0): first defined here

Fixes:
 - http://autobuild.buildroot.org/results/01b22faf08a798ecc5d5465e1c0c13fa024f1096



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