lib+common: Fix v.h linkage for dynamically linked build

`make lib` builds c-ray in two parts, `lib/libc-ray.so` and `lib/c-ray`.
When I added v.h initially, I just placed the symbols in
`lib/api/c-ray.c`, but doing it that way failed to export the symbols
for the driver portion in `lib/c-ray`. Fix is to put the symbols in a
new translation unit under `src/common`.

Author: vkoskiv

src/common/v_impl.c

@@ -0,0 +1,2 @@
+#define V_IMPLEMENTATION
+#include <v.h>

src/lib/api/c-ray.c

@@ -13,7 +13,6 @@
 #include <stdbool.h>
 #include <string.h>
 
-#define V_IMPLEMENTATION
 #include <v.h>
 
 #include <common/gitsha1.h>