samples: bpf: Fix linking xdp_router_ipv4 after migration

Users of the xdp_sample_user infra should be explicitly linked
with the standard math library (`-lm`). Otherwise, the following
happens:

/usr/bin/ld: xdp_sample_user.c:(.text+0x59fc): undefined reference to `ceil'
/usr/bin/ld: xdp_sample_user.c:(.text+0x5a0d): undefined reference to `ceil'
/usr/bin/ld: xdp_sample_user.c:(.text+0x5adc): undefined reference to `floor'
/usr/bin/ld: xdp_sample_user.c:(.text+0x5b01): undefined reference to `ceil'
/usr/bin/ld: xdp_sample_user.c:(.text+0x5c1e): undefined reference to `floor'
/usr/bin/ld: xdp_sample_user.c:(.text+0x5c43): undefined reference to `ceil
[...]

That happened previously, so there's a block of linkage flags in the
Makefile. xdp_router_ipv4 has been transferred to this infra quite
recently, but hasn't been added to it. Fix.

Fixes: 85bf1f51 ("samples: bpf: Convert xdp_router_ipv4 to XDP samples helper")
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220404115451.1116478-1-alexandr.lobakin@intel.com

samples/bpf/Makefile

@@ -219,6 +219,7 @@ TPROGLDLIBS_xdp_redirect	+= -lm
 TPROGLDLIBS_xdp_redirect_cpu	+= -lm
 TPROGLDLIBS_xdp_redirect_map	+= -lm
 TPROGLDLIBS_xdp_redirect_map_multi += -lm
+TPROGLDLIBS_xdp_router_ipv4	+= -lm
 TPROGLDLIBS_tracex4		+= -lrt
 TPROGLDLIBS_trace_output	+= -lrt
 TPROGLDLIBS_map_perf_test	+= -lrt