Replace \e with \x1B, as the former is a gcc extension

While supported by gcc and clang, \e is a gcc-specific extension and not
formally defined in the C or C++ standards.

See [0] for a list of valid escapes.

[0]: https://stackoverflow.com/a/10220539/17027

src/input_common.cpp

@@ -29,7 +29,7 @@
 #include "wutil.h"
 
 /// Time in milliseconds to wait for another byte to be available for reading
-/// after \e is read before assuming that escape key was pressed, and not an
+/// after \x1B is read before assuming that escape key was pressed, and not an
 /// escape sequence.
 #define WAIT_ON_ESCAPE_DEFAULT 300
 static int wait_on_escape_ms = WAIT_ON_ESCAPE_DEFAULT;