manual: Add missing vim header

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/customize-outside-br.txt

@@ -1,4 +1,5 @@
 // -*- mode:doc -*- ;
+// vim: set syntax=asciidoc:
 
 [[outside-br-custom]]
 === Keeping customizations outside of Buildroot

docs/manual/customize-patches.txt

@@ -1,4 +1,5 @@
 // -*- mode:doc -*- ;
+// vim: set syntax=asciidoc:
 
 [[customize-patches]]
 === Adding project-specific patches

docs/manual/makeusers-syntax.txt

@@ -1,4 +1,5 @@
 // -*- mode:doc -*- ;
+// vim: set syntax=asciidoc:
 
 [[makeuser-syntax]]
 == Makeusers syntax documentation