CODING_STYLE: Section about mixed declarations

We had an unwritten rule about declarations having to be at beginning of
blocks. Make it a written rule.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

CODING_STYLE

@@ -84,3 +84,10 @@ and clarity it comes on a line by itself:
 Rationale: a consistent (except for functions...) bracing style reduces
 ambiguity and avoids needless churn when lines are added or removed.
 Furthermore, it is the QEMU coding style.
+
+5. Declarations
+
+Mixed declarations (interleaving statements and declarations within blocks)
+are not allowed; declarations should be at the beginning of blocks.  In other
+words, the code should not generate warnings if using GCC's
+-Wdeclaration-after-statement option.