fix(esp_libc): provide picolibc timespec_get

The Picolibc headers in the ESP-IDF v6.0 toolchain declare the C11
timespec_get API, but the corresponding archives do not define it.
Provide the missing compatibility implementation through esp_libc
and cover valid and invalid time bases.

components/esp_libc/src/picolibc/timespec_get.c

@@ -0,0 +1,15 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <time.h>
+
+int timespec_get(struct timespec *ts, int base)
+{
+    if (base != TIME_UTC || clock_gettime(CLOCK_REALTIME, ts) < 0) {
+        return 0;
+    }
+    return base;
+}