libcanfestival: fix build on LEDE buildbots using ccache

libcanfestival: fix build on LEDE buildbots using ccache

This error is reported:

make -C can_socket driver
make[6]: Entering directory '/data/bowl-builder/arm_arm926ej-s/build/sdk/build_dir/target-arm_arm926ej-s_musl-1.1.15_eabi/libcanfestival/drivers/can_socket'
ccache_cc -O2 -fPIC -DDEBUG_ERR_CONSOLE_ON -g  -I../../include -I../../include/unix -I../../include/can_socket -o can_socket.o -c can_socket.c
cc1: note: someone does not honour COPTS correctly, passed 0 times
ccache_cc -shared -Wl,-soname,libcanfestival_can_socket.so  -o libcanfestival_can_socket.so can_socket.o
make[6]: Leaving directory '/data/bowl-builder/arm_arm926ej-s/build/sdk/build_dir/target-arm_arm926ej-s_musl-1.1.15_eabi/libcanfestival/drivers/can_socket'
make -C unix driver
make[6]: Entering directory '/data/bowl-builder/arm_arm926ej-s/build/sdk/build_dir/target-arm_arm926ej-s_musl-1.1.15_eabi/libcanfestival/drivers/unix'
ccache_cc -O2 -DDEBUG_ERR_CONSOLE_ON -g  -I../../include -I../../include/unix -I../../include/timers_unix -o unix.o -c unix.c
cc1: note: someone does not honour COPTS correctly, passed 0 times
Building [libcanfestival_unix.a]
ccache_ccar rc libcanfestival_unix.a unix.o ../timers_unix/timers_unix.o
make[6]: ccache_ccar: Command not found
Makefile:102: recipe for target 'libcanfestival_unix.a' failed
make[6]: *** [libcanfestival_unix.a] Error 127
make[6]: Leaving directory '/data/bowl-builder/arm_arm926ej-s/build/sdk/build_dir/target-arm_arm926ej-s_musl-1.1.15_eabi/libcanfestival/drivers/unix'
Makefile:33: recipe for target 'driver' failed
make[5]: *** [driver] Error 2

Signed-off-by: Michael Heimpold <redacted>