gdb-add-index: quote PKGVERSION

gdb-add-index: quote PKGVERSION

In Gentoo, we configure our gdb with `--with-pkgversion=` with
"Gentoo VERSION XXXX" where XXX depends on patching (not that we patch
gdb really these days) or vanilla.

Since 71f193a5c1cb02dcde6ac160cdab88e9725862bb, this goes wrong, yielding
```
/usr/bin/gdb-add-index: 25: Syntax error: "(" unexpected
```

with lines 25-26 being:
```
PKGVERSION=(Gentoo 9999 vanilla)
VERSION=17.0.50.20250319-git
```

Quote both assignments (PKGVERSION by necessity, VERSION for consistency
or symmetry).

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32325