Add missing include with EINVAL definition

[zaporozhets]

Hi,

The following PR fixes open-amp compilation with Vitis 2023.2 for cortex-R5F core. The compilation process stops with two errors in rpmsg_rpc_server.c and rpmsg_rpc_client.c due to undeclared EINVAL.

Let me know if you have any suggestions on this patch or better ideas on how to fix this issue(maybe in libmetal).

Please see the logs attached.

 /YOYO//workspace/ultra96_platform/psu_cortexr5_0/freertos_psu_cortexr5_0/bsp/include/xparameters.h:228: note: this is the location of the previous definition
       | 
 /YOYO//workspace/ultra96_platform/psu_cortexr5_0/freertos_psu_cortexr5_0/bsp/libsrc/openamp/src/lib/service/rpmsg/rpc/rpmsg_rpc_server.c: In function 'rpmsg_endpoint_server_cb':
[ERROR] /YOYO//workspace/ultra96_platform/psu_cortexr5_0/freertos_psu_cortexr5_0/bsp/libsrc/openamp/src/lib/service/rpmsg/rpc/rpmsg_rpc_server.c:61:25: error: 'EINVAL' undeclared (first use in this function)
       |                         ^~~~~~
 /YOYO//workspace/ultra96_platform/psu_cortexr5_0/freertos_psu_cortexr5_0/bsp/libsrc/openamp/src/lib/service/rpmsg/rpc/rpmsg_rpc_server.c:61:25: note: each undeclared identifier is reported only once for each function it appears in
 /YOYO//workspace/ultra96_platform/psu_cortexr5_0/freertos_psu_cortexr5_0/bsp/libsrc/openamp/src/lib/service/rpmsg/rpc/rpmsg_rpc_server.c: In function 'rpmsg_rpc_server_send':
[ERROR] /YOYO//workspace/ultra96_platform/psu_cortexr5_0/freertos_psu_cortexr5_0/bsp/libsrc/openamp/src/lib/service/rpmsg/rpc/rpmsg_rpc_server.c:88:25: error: 'EINVAL' undeclared (first use in this function)
     

[tnmysh]

@arnopo, @edmooring for reviews.