src/dpdk_telemetry.c: fix build on musl

src/dpdk_telemetry.c: fix build on musl

Include unistd.h instead of sys/unistd.h to fix the following build
failure on musl:

src/dpdk_telemetry.c:43:10: fatal error: sys/unistd.h: No such file or directory
 #include <sys/unistd.h>
          ^~~~~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/aafb8c72f147fefc7a988c45e4dc17de48b07a95

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>