netpbm: Depends on flex, zlib for Linuxbrew
netpbm: Depends on flex, zlib for Linuxbrew

Closes Linuxbrew/homebrew-core#413.

Signed-off-by: Shaun Jackman <b580dab3251a9622aba3803114310c23fdb42900@gmail.com>

Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core,maxim-belkin/homebrew-core,rwhogg/homebrew-core,Linuxbrew/homebrew-core