Revert "Install everything in the `include/` directory into `$(prefix)/include/openlibm`."

This reverts commit eb2f4d4b44e6ffaede3057d2cb6c6bae77de863f.

Makefile

@@ -43,7 +43,6 @@ distclean:
 
 install: all
 	mkdir -p $(DESTDIR)$(libdir)
-	mkdir -p $(DESTDIR)$(includedir)/openlibm
+	mkdir -p $(DESTDIR)$(includedir)
 	cp -a libopenlibm.$(SHLIB_EXT)* libopenlibm.a $(DESTDIR)$(libdir)/
 	cp -a src/openlibm.h $(DESTDIR)$(includedir)/
-	cp -a include/*.h $(DESTDIR)$(includedir)/openlibm/