Miracl JNI: include string.h to get memcpy

It failed like that:

>make
Compiling the Miracl jni interface...
make[1]: Entering directory `/home/muelli/vcs/scapi/src/jni/MiraclJavaInterface'
g++ -DNDEBUG -g -O2 -fPIC -DMR_GENERIC_MT -c Dlog.c -I/home/muelli/vcs/scapi/install/include/miracl -I/usr/lib/jvm/java-7-openjdk-amd64/include/ -I/usr/lib/jvm/java-7-openjdk-amd64/include/linux/
Dlog.c: In function ‘jlong Java_edu_biu_scapi_primitives_dlog_miracl_MiraclDlogECFp_encodeByteArrayToPoint(JNIEnv*, jobject, jlong, jbyteArray, jint)’:
Dlog.c:567:39: error: ‘memcpy’ was not declared in this scope
    memcpy(newString+l-k-2, string, len);
                                       ^
make[1]: *** [Dlog.o] Error 1
make[1]: Leaving directory `/home/muelli/vcs/scapi/src/jni/MiraclJavaInterface'
make: *** [src/jni/MiraclJavaInterface/libMiraclJavaInterface.so] Error 2