Fix the build failure (sonic-net#24602)

Why I did it
Regression due to sonic-net#23893

Work item tracking
Microsoft ADO (number only):
sonic-net#24601
How I did it
Added the missing declaration of the referenced variable in the function fpm_nl_enqueue

How to verify it
Rebuilt the image, the compilation error was no longer seen.

Author: vivekverma-arista

src/sonic-frr/dplane_fpm_sonic/dplane_fpm_sonic.c

@@ -2445,6 +2445,7 @@ static int fpm_nl_enqueue(struct fpm_nl_ctx *fnc, struct zebra_dplane_ctx *ctx)
 	ssize_t rv;
 	uint64_t obytes, obytes_peak;
 	enum dplane_op_e op = dplane_ctx_get_op(ctx);
+	struct nexthop *nexthop;
 
 	/*
 	 * If we were configured to not use next hop groups, then quit as soon