gsoap-native: add missing dependency on flex-native

* otherwise fails with:
| gcc  -DHAVE_CONFIG_H -I. -I/work/x86_64-linux/gsoap-native/2.8.12-r1/gsoap-2.8/gsoap/src -I../..   -isystem/sysroots/x86_64-linux/usr/include -DWITH_BISON -DWITH_LEX  -DSOAPCPP_IMPORT_PATH="\"/sysroots/x86_64-linux/usr/share/gsoap/import\"" -DLINUX -isystem/sysroots/x86_64-linux/usr/include -O2 -pipe -c -o soapcpp2-soapcpp2_yacc.o `test -f 'soapcpp2_yacc.c' || echo '/work/x86_64-linux/gsoap-native/2.8.12-r1/gsoap-2.8/gsoap/src/'`soapcpp2_yacc.c
| /bin/bash /work/x86_64-linux/gsoap-native/2.8.12-r1/gsoap-2.8/ylwrap /work/x86_64-linux/gsoap-native/2.8.12-r1/gsoap-2.8/gsoap/src/soapcpp2_lex.l .c soapcpp2_lex.c -- /bin/bash /work/x86_64-linux/gsoap-native/2.8.12-r1/gsoap-2.8/missing flex
| /work/x86_64-linux/gsoap-native/2.8.12-r1/gsoap-2.8/missing: line 81: flex: command not found
| WARNING: 'flex' is missing on your system.
|          You should only need it if you modified a '.l' file.
|          You may want to install the Fast Lexical Analyzer package:
|          <http://flex.sourceforge.net/

>
| make[4]: *** [soapcpp2_lex.c] Error 127
| make[4]: Leaving directory `/work/x86_64-linux/gsoap-native/2.8.12-r1/build/gsoap/src'
| make[3]: *** [all-recursive] Error 1

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>