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/COPYING.LIB

@@ -133,7 +133,7 @@ such a program is covered only if its contents constitute a work based
 on the Library (independent of the use of the Library in a tool for
 writing it).  Whether that is true depends on what the Library does
 and what the program that uses the Library does.
-  
+
   1. You may copy and distribute verbatim copies of the Library's
 complete source code as you receive it, in any medium, provided that
 you conspicuously and appropriately publish on each copy an

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