Link to libm for truncf

Weird - I do not understand why this did not pop up earlier?

Conflicts:
	src/Makefile.am

src/Makefile.am

@@ -208,7 +208,7 @@ noinst_LTLIBRARIES += \
 	libfellowwitness.la
 
 libfellow_libadd = \
-	$(VARNISHSRC)/lib/libvarnish/libvarnish.la $(XXHASH_LIBS)
+	$(VARNISHSRC)/lib/libvarnish/libvarnish.la $(XXHASH_LIBS) -lm
 
 io_ioctl_sources = \
 	fellow_io_ioctl.c \