[posix] clear pending alarms in posix platform which could persist execve call (#2323)

diff --git a/examples/platforms/posix/misc.c b/examples/platforms/posix/misc.c
index 38d41ee..ec0b6d7 100644
--- a/examples/platforms/posix/misc.c
+++ b/examples/platforms/posix/misc.c
@@ -55,6 +55,8 @@
     platformRadioDeinit();
     platformUartRestore();
 
+    alarm(0);
+
     execvp(argv[0], argv);
     perror("reset failed");
     exit(EXIT_FAILURE);