nsresourced: Fix declaration of bpf_rdonly_cast()

nsresourced: Fix declaration of bpf_rdonly_cast()

Fixes compilation error

"""
[780/3171] /usr/bin/clang -std=gnu11 -Wno-compare-distinct-pointer-types -fno-stack-protector -O2 -target bpf -g -c -D__aarch64__ -I. -isystem /usr/include/ -idirafter /usr/include ../src/nsresourced/bpf/userns_restrict/userns-restrict.bpf.c -o src/nsresourced/bpf/userns_restrict/userns-restrict.bpf.unstripped.o -I/usr/src/kernels/6.11.1-0.hs1.hs+fb.el9.aarch64
FAILED: src/nsresourced/bpf/userns_restrict/userns-restrict.bpf.unstripped.o
/usr/bin/clang -std=gnu11 -Wno-compare-distinct-pointer-types -fno-stack-protector -O2 -target bpf -g -c -D__aarch64__ -I. -isystem /usr/include/ -idirafter /usr/include ../src/nsresourced/bpf/userns_restrict/userns-restrict.bpf.c -o src/nsresourced/bpf/userns_restrict/userns-restrict.bpf.unstripped.o -I/usr/src/kernels/6.11.1-0.hs1.hs+fb.el9.aarch64
../src/nsresourced/bpf/userns_restrict/userns-restrict.bpf.c:27:7: error: conflicting types for 'bpf_rdonly_cast'
   27 | void *bpf_rdonly_cast(void *, __u32) __ksym;
      |       ^
/usr/src/kernels/6.11.1-0.hs1.hs+fb.el9.aarch64/vmlinux.h:143063:14: note: previous declaration is here
 143063 | extern void *bpf_rdonly_cast(const void *obj__ign, u32 btf_id__k) __weak __ksym;
        |              ^
1 error generated.
"""