backends/epson.c needs a MAX macro when i build on Fedora30/x86_64

[flachs]

i noticed in my syslog that the epson backend was not loading due to missing symbol MAX.

i don't know where MAX was to be defined. I searched all .h files for a definition and there isnt one in the project. Version 0.2.3 had a larger buffer and didnt check for buffer overflow.

i dropped the following in backends/epson.c and it builds and runs now.

#ifndef MAX
#define MAX(a,b) ((a)>(b) ? (a) : (b))
#endif