Fix undefined reference to libm functions linking tor-fw-helper

When configuring tor without upnp support, ie ./configure --disable-upnp,
tor-fw-helper fails to link with undefined references to `ceil' and
`log'.  This if fixed by linking to libm.

X-Gentoo-Bug: 435040
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=435040
Reported-by: Alexandre <alexandre.cortes@outlook.com>
Signed-off-by: Anthony G. Basile <blueness@gentoo.org>

src/tools/tor-fw-helper/include.am

@@ -23,7 +23,7 @@ endif
 
 if MINIUPNPC
 miniupnpc_ldflags = @TOR_LDFLAGS_libminiupnpc@
-miniupnpc_ldadd = -lminiupnpc -lm @TOR_LIB_IPHLPAPI@
+miniupnpc_ldadd = -lminiupnpc @TOR_LIB_IPHLPAPI@
 miniupnpc_cppflags = @TOR_CPPFLAGS_libminiupnpc@
 else
 miniupnpc_ldflags =
@@ -32,5 +32,5 @@ miniupnpc_cppflags =
 endif
 
 src_tools_tor_fw_helper_tor_fw_helper_LDFLAGS = $(nat_pmp_ldflags) $(miniupnpc_ldflags)
-src_tools_tor_fw_helper_tor_fw_helper_LDADD = src/common/libor.a $(nat_pmp_ldadd) $(miniupnpc_ldadd) @TOR_LIB_WS32@
+src_tools_tor_fw_helper_tor_fw_helper_LDADD = src/common/libor.a $(nat_pmp_ldadd) $(miniupnpc_ldadd) -lm @TOR_LIB_WS32@
 src_tools_tor_fw_helper_tor_fw_helper_CPPFLAGS = $(nat_pmp_cppflags) $(miniupnpc_cppflags)