Remove trailing whitespace.

Also, the script is a one-liner, stolen from [1]:

ack --print0 -l '[ \t]+$' | xargs -0 -n1 perl -pi -e 's/[ \t]+$//'

[1]
https://stackoverflow.com/questions/149057/how-to-remove-trailing-whitespace-of-all-files-recursively

wmlib/Makefile.am

@@ -16,7 +16,7 @@ libWMaker_la_SOURCES = 	\
 	event.c 	\
 	command.c	\
 	app.h		\
-	menu.h	
+	menu.h
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = wmlib.pc