Move source files from `src/vixl/` to `src/`.

This commit only moves files and updates the `#include` paths appropriately.

Change-Id: I9d430bd34af617c943a6c6570c9a3c27169eaf1b
diff --git a/examples/custom-disassembler.h b/examples/custom-disassembler.h
index 4ea1da7..cdf6b1a 100644
--- a/examples/custom-disassembler.h
+++ b/examples/custom-disassembler.h
@@ -27,7 +27,7 @@
 #ifndef VIXL_EXAMPLES_CUSTOM_DISASSEMBLER_H_
 #define VIXL_EXAMPLES_CUSTOM_DISASSEMBLER_H_
 
-#include "vixl/a64/disasm-a64.h"
+#include "a64/disasm-a64.h"
 
 using namespace vixl;