Parse packages sizes >=1GB

Some packages have now become too big. The kernel-source package is
1.1GB installed. When installed with pkgtools, its size is now stated as
"1.1G" in the UNCOMPRESSED SIZE field in the respective
/var/lib/pkgtools/packages file.

There's also another issue that this change doesn't handle. In some
languages the decimal point is a "," instead of a "." and that's what
the numfmt tool, which is used by pkgtools, uses. So the size is stated
as "1,1G" instead. The C sscanf function we're using here doesn't parse
that correctly, leaving the decimal part out. So a package of size
"1,9G" would end up showing as 1024M. Same issue with packages of size
>1MB and <10MB, they would show up as "1,1M" for example. This doesn't
create any actual problems with installing/removing/upgrading packages
through. The discrepancy is only showed when running spkg --list. I'll
leave it like this for now.

src/pkgdb.c

@@ -651,6 +651,8 @@ static gint _parse_size(const char* str, guint* size)
       *size = (guint)v;
     else if (strchr(str, 'M'))
       *size = (guint)(v * 1024);
+    else if (strchr(str, 'G'))
+      *size = (guint)(v * 1024 * 1024);
     else
       return 0;