package/freerdp: fix build with libexecinfo

Fix the following build failure raised on uclibc and musl since the
addition of libexecinfo package in commit
eea8ba44:

/home/buildroot/autobuild/instance-0/output-1/host/lib/gcc/riscv64-buildroot-linux-uclibc/10.3.0/../../../../riscv64-buildroot-linux-uclibc/bin/ld: ../../libwinpr/libwinpr2.so.2.5.0: undefined reference to `backtrace_symbols_fd'

Fixes:
 - http://autobuild.buildroot.org/results/095c33098a6e59ff664080e03baf1a3c92b4265f



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>