error: fix undeclared memcpy

[0xc0170]

Description

Some targets do not get string header file in, results in the warning:
implicit declaration of function 'memcpy'

I discovered this while compiling mbed 2 for RZ_A1H target

Compile [ 84.7%]: mbed_error.c
[Warning] mbed_error.c@117,5: implicit declaration of function 'memset' [-Wimplicit-function-declaration]
[Warning] mbed_error.c@145,9: implicit declaration of function 'memcpy' [-Wimplicit-function-declaration]

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2251
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7136/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 2040
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7136/2040

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1876
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7136/