Specify OSX deployment target for python package. (#3194) (#3279)

Summary:
Pull Request resolved: #3194
overriding_review_checks_triggers_an_audit_and_retroactive_review
Oncall Short Name: executorch

Differential Revision: D56405473

fbshipit-source-id: 785709e8acc1b07e57825b278c3e0a355641e13a
(cherry picked from commit a7a9ab3)

Co-authored-by: Anthony Shoumikhin <shoumikhin@meta.com>

setup.py

@@ -429,6 +429,7 @@ def run(self):
             # useful error information to users.
             "-DEXECUTORCH_ENABLE_LOGGING=ON",
             "-DEXECUTORCH_LOG_LEVEL=Info",
+            "-DCMAKE_OSX_DEPLOYMENT_TARGET=10.15",
         ]
 
         build_args = [f"-j{self.parallel}"]