Add a missed DESTDIR

Signed-off-by: Mattias Andrée <maandree@kth.se>

Makefile

@@ -17,7 +17,7 @@ target/release/exa:
 
 install: target/release/exa
 	# BSD and OSX don't have -D to create leading directories
-	install -dm755 -- "$(PREFIX)/bin/" "$(DESTDIR)$(PREFIX)/share/man/man1/"
+	install -dm755 -- "$(DESTDIR)$(PREFIX)/bin/" "$(DESTDIR)$(PREFIX)/share/man/man1/"
 	install -sm755 -- target/release/exa "$(DESTDIR)$(PREFIX)/bin/"
 	install -m644  -- contrib/man/exa.1 "$(DESTDIR)$(PREFIX)/share/man/man1/"