unirec: Mark usage of statement expressions as such

[krkos]

• The use of statement expressions is forbidden by ANSI C but is an GCC extension. It is used in unirec.h to define the TRAP_CTX_RECEIVE and (commented out) UNIREC_CTX_CHECK_TEMPLATE macros. By marking these declarations as extension, several build warnings are fixed.
• For further details: https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
• This should also be supported by Clang, not tested.

[codecov-io]

Codecov Report

Merging #140 into master will increase coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
+ Coverage   51.84%   51.88%   +0.03%     
==========================================
  Files          51       51              
  Lines       10633    10633              
==========================================
+ Hits         5513     5517       +4     
+ Misses       5120     5116       -4

Impacted Files	Coverage Δ
libtrap/tests/test_echo_reply.c	82.69% <0%> (+7.69%)	:arrow_up:

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[cejkato2]

@havraji6 could you please check this update on turris and openwrt?