Fix a compilation failure in bio_comp_test.c
Compiling with clang, --strict-warnings and enable-zlib-dynamic resulted
in a compilation failure. This fixes it.
static int sizes[NUM_SIZES] = { 64, 512, 2048, 16 * 1024 };
/* using global buffers */
-unsigned char *original = NULL;
-unsigned char *result = NULL;
+static unsigned char *original = NULL;
+static unsigned char *result = NULL;
/*
* For compression:
