package/lttng-babeltrace: fix popt static build with libiconv

Fix the following popt static build failure with libiconv:

configure:18149: checking for poptGetContext in -lpopt
configure:18172: /home/buildroot/autobuild/instance-2/output-1/host/bin/m68k-buildroot-linux-uclibc-gcc -o conftest -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os -g0  -static  -I/home/buildroot/autobuild/instance-2/output-1/host/bin/../m68k-buildroot-linux-uclibc/sysroot/usr/include/uuid  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -static conftest.c -lpopt  -L/home/buildroot/autobuild/instance-2/output-1/host/bin/../m68k-buildroot-linux-uclibc/sysroot/usr/lib -luuid    >&5
/home/buildroot/autobuild/instance-2/output-1/host/lib/gcc/m68k-buildroot-linux-uclibc/10.4.0/../../../../m68k-buildroot-linux-uclibc/bin/ld: /home/buildroot/autobuild/instance-2/output-1/host/bin/../m68k-buildroot-linux-uclibc/sysroot/usr/lib/libpopt.a(poptint.o): in function `POPT_fprintf':
poptint.c:(.text+0x28a): undefined reference to `libiconv_open'

[...]

checking for poptGetContext in -lpopt... no
configure: error: Cannot find popt.

Fixes:
 - http://autobuild.buildroot.org/results/4f465fe15664b1f9df3f26c331dc550852972f2f



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