Add missing unistd include in zlib preventing zlib build on Linux

[fran-retfie]

• Add .gitignore to prevent build folder to be tracked
• Add "unistd.h" include to "libs/zlib/gzlib.c", "libs/zlib/gzread.c" and "libs/zlib/gzwrite.c" that provides lseek, close, write and read functions