make blockchain-opencl non-plug

This fixes building on systems without OpenCL (with non-opencl make
targets used).

Compile error was:
$ make linux-x86-64i
...
gcc -c -Wall -Wdeclaration-after-statement -O2
-fomit-frame-pointer -I/usr/local/include -I/usr/include/nss
-I/usr/include/nspr -DHAVE_NSS -DHAVE_GMP -DHAVE_CRYPT -DHAVE_DL
-DUSING_ICC_S_FILE -funroll-loops opencl_blockchain_fmt_plug.c
In file included from opencl_blockchain_fmt_plug.c:29:0:
common-opencl.h:22:19: fatal error: CL/cl.h: No such file or directory
compilation terminated.
make[1]: *** [opencl_blockchain_fmt_plug.o] Error 1
make[1]: Leaving directory `/home/src/john/src'
make: *** [linux-x86-64i] Error 2