fix(build): fix build by enabling the POSIX API

 ### Summary

 Zephyr [recently deprecated](zephyrproject-rtos/zephyr#69950)
 the `CONFIG_NET_SOCKETS_POSIX_NAMES` option, which was was previously
 `y` by default when `CONFIG_POSIX_API=n` but is now n by default.
 This option matters because it adds the POSIX API to the `net/socket.h`
 header, which we reference in `memfault_platform_http.c`. This fix
 instead makes our http client code include the `posix/*` headers to get
 the POSIX API.

 ### Test Plan

 Builds successfully.

Author: gminn

overlay-http.conf

@@ -1,6 +1,7 @@
 # Memfault internet settings
 CONFIG_MEMFAULT_HTTP_ENABLE=y
 CONFIG_MEMFAULT_HTTP_PERIODIC_UPLOAD=y
+CONFIG_POSIX_API=y
 
 # Network settings
 CONFIG_NETWORKING=y