cmake: disable `-Wdeclaration-after-statement`

Disable `-Wdeclaration-after-statement`.

Before ISO C99, declaring variables after statements was not allowed
by the C standard, but was allowed by GCC as an extension.

CMocka now requires a minimum version of C99,
since commit 5a4b1587

, so this
behaviour is valid on all compilers.

on-behalf-of: @nqminds <info@nqminds.com>

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>