Add missing header needed for 'memset'.

Lack of this include would lead to misdiagnostics by libfabric's 'configure' script:

  checking whether cxil_svc_get_vni_range is declared... no

... where the real problem is lack of a 'memset' declaration:

  configure:37208: checking whether cxil_svc_get_vni_range is declared
  configure:37208: gcc -c -O2 -DNDEBUG -pipe -fvisibility=hidden -Wall -Wundef -Wpointer-arith -I/include    conftest.c >&5
  In file included from /gnu/store/…-libcxi-14.0.0/include/libcxi/libcxi.h:22,
		   from conftest.c:134:
  /gnu/store/…-cassini-headers-14.0.0/include/cxi_prov_hw.h: In function 'cxi_cq_update_wp':
  /gnu/store/…-cassini-headers-14.0.0/include/cxi_prov_hw.h:638:17: error: implicit declaration of function 'memset' [-Wimplicit-function-declaration]

--- a/include/cxi_prov_hw.h
+++ b/include/cxi_prov_hw.h
@@ -11,6 +11,7 @@
 #ifndef __KERNEL__
 #include <stdbool.h>
 #include <errno.h>
+#include <string.h>
 #endif
 
 /*