package/perl-db-file: Fix berkeleydb detection

Without this patch libdb.so is not detected and
lib/perl5/site_perl/5.22.0/i586-linux/auto/DB_File/DB_File.so
is not linked against it:

Parsing config.in...
Looks Good.
Checking if your kit is complete...
Looks good
Warning (mostly harmless): No library found for -ldb
Generating a Unix-style Makefile
Writing Makefile for DB_File
Writing MYMETA.yml and MYMETA.json
>>> perl-db-file 1.835 Building

$ output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a output/target/usr/lib/perl5/site_perl/5.22.0/i586-linux/auto/DB_File/DB_File.so | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libc.so.1]

After applying this patch configure output will look like this:

Parsing config.in...
Looks Good.
Checking if your kit is complete...
Looks good
Generating a Unix-style Makefile
Writing Makefile for DB_File
Writing MYMETA.yml and MYMETA.json
>>> perl-db-file 1.835 Building

and the resulting library is correctly linked against libdb:

$ output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a output/target/usr/lib/perl5/site_perl/5.22.0/i586-linux/auto/DB_File/DB_File.so | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libdb-5.3.so]
 0x00000001 (NEEDED)                     Shared library: [libc.so.1]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>