[FRR]: Fix compiler warnings (#23181)

When compiling FRR, the compiler raises the following warnings:

```
../zebra/dplane_fpm_sonic.c:73:1: warning: data definition has no type or storage class
 73 | DEFAULT_SRV6_LOCALSID_FORMAT_BLOCK_LEN = 32;
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../zebra/dplane_fpm_sonic.c:73:1: warning: type defaults to 'int' in declaration of 'DEFAULT_SRV6_LOCALSID_FORMAT_BLOCK_LEN' [-Wimplicit-int] ../zebra/dplane_fpm_sonic.c:74:1: warning: data definition has no type or storage class
 74 | DEFAULT_SRV6_LOCALSID_FORMAT_NODE_LEN = 16;
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../zebra/dplane_fpm_sonic.c:74:1: warning: type defaults to 'int' in declaration of 'DEFAULT_SRV6_LOCALSID_FORMAT_NODE_LEN' [-Wimplicit-int] ../zebra/dplane_fpm_sonic.c:75:1: warning: data definition has no type or storage class
 75 | DEFAULT_SRV6_LOCALSID_FORMAT_FUNCTION_LEN = 16;
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../zebra/dplane_fpm_sonic.c:75:1: warning: type defaults to 'int' in declaration of 'DEFAULT_SRV6_LOCALSID_FORMAT_FUNCTION_LEN' [-Wimplicit-int] ../zebra/dplane_fpm_sonic.c:76:1: warning: data definition has no type or storage class
 76 | DEFAULT_SRV6_LOCALSID_FORMAT_ARGUMENT_LEN = 0;
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../zebra/dplane_fpm_sonic.c:76:1: warning: type defaults to 'int' in declaration of 'DEFAULT_SRV6_LOCALSID_FORMAT_ARGUMENT_LEN' [-Wimplicit-int]
```

The problem is that some variables have been defined without specifying their type.

This commit fixes the warnings by defining those variables as macros.

Author: mssonicbld

src/sonic-frr/dplane_fpm_sonic/dplane_fpm_sonic.c

@@ -75,10 +75,10 @@
 #define FPM_HEADER_SIZE 4
 
 /* Default SRv6 SID format values */
-DEFAULT_SRV6_LOCALSID_FORMAT_BLOCK_LEN = 32;
-DEFAULT_SRV6_LOCALSID_FORMAT_NODE_LEN = 16;
-DEFAULT_SRV6_LOCALSID_FORMAT_FUNCTION_LEN = 16;
-DEFAULT_SRV6_LOCALSID_FORMAT_ARGUMENT_LEN = 0;
+#define DEFAULT_SRV6_LOCALSID_FORMAT_BLOCK_LEN 32
+#define DEFAULT_SRV6_LOCALSID_FORMAT_NODE_LEN 16
+#define DEFAULT_SRV6_LOCALSID_FORMAT_FUNCTION_LEN 16
+#define DEFAULT_SRV6_LOCALSID_FORMAT_ARGUMENT_LEN 0
 
 /*
  * Time in seconds that if the other end is not responding