libc/newlib: Add -D_POSIX_C_SOURCE=200809L to zephyr compile definitions

Newlib doesn't have Zephyr support, so the only way to get it to expose the
full Zephyr C library API is to have it expose all of the POSIX API.

Signed-off-by: Keith Packard <keithp@keithp.com>

lib/libc/newlib/CMakeLists.txt

@@ -73,6 +73,9 @@ if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
 endif()
 zephyr_link_libraries(c)
 
+# newlib needs this to expose the full Zephyr C Library API
+zephyr_compile_definitions(_POSIX_C_SOURCE=200809L)
+
 if(CONFIG_NEWLIB_LIBC_NANO)
   zephyr_link_libraries(
     -specs=nano.specs