Update Linux build dependencies: FreeImage

Author: q4a

doc/howto/build-linux.txt

@@ -2,6 +2,7 @@ TITLE: X-Ray 1.6 build and setup on Linux
 
 Required packages:
 ------
+FreeImage
 LockFile
 OpenAL
 TBB
@@ -15,7 +16,7 @@ JPEG
 
 
 Ubuntu 18.04 build dependencies:
-sudo apt install git cmake liblockfile-dev libopenal-dev libtbb-dev libcrypto++-dev libpugixml-dev libogg-dev libtheora-dev libvorbis-dev libsdl2-dev liblzo2-dev libjpeg-dev libncurses5-dev
+sudo apt install git cmake libfreeimage-dev liblockfile-dev libopenal-dev libtbb-dev libcrypto++-dev libpugixml-dev libogg-dev libtheora-dev libvorbis-dev libsdl2-dev liblzo2-dev libjpeg-dev libncurses5-dev
 
 Build commands:
 ------