package/docopt-cpp: fix static build

Fix the following static build failure raised since the addition of the
package in commit 43f96ced67c9a1f84d3f0edf5cd632178eb176b8:

[ 66%] Linking CXX shared library libdocopt.so
ld (ld-elf2flt): -shared used without passing a shared library ID

Fixes:
 - http://autobuild.buildroot.org/results/834a0209c8165b3208ebf8654cb6cf8e3568b671

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