Portability: Fixed broken build for Cygwin and older GCC

- Thanks to Richard Boß.

ChangeLog

@@ -5,6 +5,7 @@
 - TW-1704 Use Task::identifier to reference the Task in the output
 - TW-1720 CmdContext uses a mix of both throw and std::cout to convey
           errors (thanks to Paul Beckingham).
+- Fixed broken build for Cygwin and older GCC (thanks to Richard Boß).
 - The default configuration is now 256-color only.
 - The 'columns' report now shows whether a column is modifiable or read only.
 

src/commands/CmdEdit.cpp

@@ -32,6 +32,7 @@
 #include <cstring>
 #include <algorithm>
 #include <unistd.h>
+#include <errno.h>
 #include <ISO8601.h>
 #include <Context.h>
 #include <Lexer.h>