fix 'depend' target in otherlibs/bigarray
fix 'depend' target in otherlibs/bigarray

This target has been broken by the move of bigarray C files to the runtime.
Before the current PR:

```
$ make depend
gcc -MM -O2 -fno-strict-aliasing -fwrapv -Wall -Werror -fno-tree-vrp -fPIC -I../unix -DIN_OCAML_BIGARRAY -D_FILE_OFFSET_BITS=64 -D_REENTRANT -DCAML_NAME_SPACE  -I../../byterun *.c | sed -e 's/\.o/.$(O)/g' > .depend
gcc: error: *.c: No such file or directory
gcc: fatal error: no input files
```

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml