Fix compilation with -Wl,--as-needed

[Thomas de Grenier de Latour]

Parted doesn't compile with LDFLAGS="-Wl,--as-needed", because
libparted.so is not linked to libdl, and thus has unresolved symbols.

mkdir .libs
gcc -D_REENTRANT -D_FILE_OFFSET_BITS=64
-DLOCALEDIR=\"/usr/local/share/locale\"
-DLOCALEDIR=\"/usr/local/share/locale\" -W -Wall -Wno-unused
-Wno-switch -Wno-format -Werror -Wl,--as-needed -o .libs/partprobe
partprobe.o  ../libparted/.libs/libparted.so -luuid -lreadline -ldl
-lncurses ../libparted/.libs/libparted.so: undefined reference to
`dlerror' ../libparted/.libs/libparted.so: undefined reference to
`dlclose' ../libparted/.libs/libparted.so: undefined reference to
`dlopen' ../libparted/.libs/libparted.so: undefined reference to
`dlsym' 
collect2: ld returned 1 exit status 
make[2]: *** [partprobe] Erreur 1 
make[2]: quittant le répertoire « /tmp/parted/partprobe »

This also breaks compilation of other tools (qparted/gparted) with this
linker flag. See this Gentoo bug report:
https://bugs.gentoo.org/show_bug.cgi?id=131841

The attached patch (for today trunk) fixes that by adding the -ldl to
libparted_LIBADD.

-- 
TGL.

parted-as-needed.patch
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