Change namespace for header files libuev/uev.h --> uev/uev.h

The `libuev/` namespace for header files was introduced in v1.2.1.  This
patch changes it to the less surprising `uev/`.  Hopefully there aren't
that many users of this library out there yet, apart from myself.

Apologies if you are affected!

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

Makefile.am

@@ -1,6 +1,7 @@
 ACLOCAL_AMFLAGS     = -I m4
 
 lib_LTLIBRARIES     = libuev.la
+pkgincludedir       = $(includedir)/uev
 pkginclude_HEADERS  = uev.h private.h queue.h
 libuev_la_SOURCES   = uev.c io.c timer.c signal.c