Fix strrchr() not declared for IAR

[ccli8]

Summary of changes

For IAR, strrchr() is used in MBED_FILENAME macro definition. Declare it explicitly to fix compile error when MBED_FILENAME is expanded.

mbed-os/platform/include/platform/mbed_toolchain.h

Lines 509 to 510 in 9bdbe9c

	#elif defined(__ICCARM__)
	#define MBED_FILENAME (strrchr(__FILE__, '/') ? strrchr(__FILE__, '/') + 1 : strrchr(__FILE__, '\\') ? strrchr(__FILE__, '\\') + 1 : __FILE__)

Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

---

Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

---

[ciarmcom]

@ccli8, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Jenkins CI Test : ✔️ SUCCESS

Build Number: 1 | 🔒 Jenkins CI Job | 🌐 Logs & Artifacts

CLICK for Detailed Summary

jobs	Status
jenkins-ci/mbed-os-ci_build-cloud-example-ARM	✔️
jenkins-ci/mbed-os-ci_build-cloud-example-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_unittests	✔️
jenkins-ci/mbed-os-ci_build-greentea-ARM	✔️
jenkins-ci/mbed-os-ci_build-greentea-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_build-example-ARM	✔️
jenkins-ci/mbed-os-ci_build-example-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_greentea-test	✔️

[0xc0170]

This bug was introduced recently, adding MBED_FILENAME to the error msg. Thanks for fixing it