[api] add missing header for definition of size_t

[wgtdkp]

size_t is not defined in some raspberry pi devices with gcc 6.

This PR includes <stddef.h> for definition of size_t.

[codecov-commenter]

Codecov Report

Merging #120 into master will increase coverage by 0.27%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
+ Coverage   61.96%   62.23%   +0.27%     
==========================================
  Files          58       58              
  Lines        5369     5368       -1     
==========================================
+ Hits         3327     3341      +14     
+ Misses       2042     2027      -15     

Impacted Files	Coverage Δ
src/app/cli/console.cpp	76.92% <0.00%> (-11.54%)	:arrow_down:
src/library/commissioner_safe.cpp	62.34% <0.00%> (-0.75%)	:arrow_down:
src/app/commissioner_app.hpp	80.00% <0.00%> (ø)
src/library/commissioner_safe.hpp	100.00% <0.00%> (ø)
include/commissioner/commissioner.hpp	43.75% <0.00%> (ø)
src/app/commissioner_app.cpp	29.30% <0.00%> (+0.21%)	:arrow_up:
src/common/error.cpp	26.08% <0.00%> (+2.17%)	:arrow_up:
src/app/border_agent.cpp	55.88% <0.00%> (+2.20%)	:arrow_up:
src/app/cli/interpreter.cpp	40.14% <0.00%> (+2.41%)	:arrow_up:
include/commissioner/error.hpp	100.00% <0.00%> (+3.70%)	:arrow_up: