Modify PKG_LDFLAGS in watch.rb

"PKG_LDFLAGS=-Wl" without following options would be a bad parameter.
Actually, this specification breaks make on Linuxbrew.
I tested this patch on both Homebrew and Linuxbrew.

```
==> make watch PKG_LDFLAGS=-Wl
    asprintf(&header, "Every %.1fs: %.*s",
            ^
/usr/bin/gcc-4.8 -fno-common -ffast-math -W -Wall -Wshadow -Wcast-align -Wredundant-decls -Wbad-function-cast -Wcast-qual -Wwrite-strings -Waggregate-return -Wstrict-prototypes -Wmissing-prototypes -O2 -s         watch.o -Wl  -o watch -lncurses
gcc-4.8: error: unrecognized command line option ‘-Wl’
make: *** [watch] Error 1
```