[gdb/testsuite] Fix timeout in gdb.ada/ghost.exp

[gdb/testsuite] Fix timeout in gdb.ada/ghost.exp

On openSUSE Tumbleweed with gcc-12, I run into a timeout:
...
(gdb) print value^M
Multiple matches for value^M
[0] cancel^M
[1] ada.strings.maps.value (<ref> ada.strings.maps.character_mapping; \
    character) return character at a-strmap.adb:599^M
[2] pck.value at src/gdb/testsuite/gdb.ada/ghost/pck.ads:17^M
[3] system.object_reader.value (<ref> system.object_reader.object_symbol) \
    return system.object_reader.uint64 at s-objrea.adb:2279^M
[4] system.traceback.symbolic.value (system.address) return string at \
    s-trasym.adb:200^M
> FAIL: gdb.ada/ghost.exp: print value (timeout)
print ghost_value^M
Argument must be choice number^M
(gdb) FAIL: gdb.ada/ghost.exp: print ghost_value
...

Fix this by prefixing value (as well as the other printed values) with the
package name:
...
(gdb) print pck.value^M
...

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29055