vimproc.vim fixed one issue about .gitignore file, so update it to avoid
an error for vimproc library missing.
The root reason is that when running `make`, the folder lib does not
exist, so it will be failed.
Following output is when I run `make` mannually.
```
$ make
make -f make_unix.mak
make[1]: Entering directory '/home/bright/.SpaceVim/bundle/vimproc.vim'
cc -W -O2 -Wall -Wno-unused -Wno-unused-parameter -std=gnu99 -pedantic -shared -fPIC -o lib/vimproc_linux64.so src/proc.c -lutil
/usr/bin/ld: cannot open output file lib/vimproc_linux64.so: No such file or directory
collect2: error: ld returned 1 exit status
make[1]: *** [make_unix.mak:17: lib/vimproc_linux64.so] Error 1
make[1]: Leaving directory '/home/bright/.SpaceVim/bundle/vimproc.vim'
make: *** [Makefile:66: all] Error 2
```
Co-authored-by: Bright Chen <bright.chen@autodesk.com>
