Fix pkg-config file to pull in libm

vpx uses symbols in libm and thus we need to provide an indication to
the user of libvpx that if they want to link against libvpx they must
also link against libm.

Change-Id: I31d4068bf7f6f5b1fd222bcdf9e6a1a92fb6696f

libs.mk

@@ -242,7 +242,7 @@ vpx.pc: config.mk libs.mk
 	$(qexec)echo 'Version: $(VERSION_MAJOR).$(VERSION_MINOR).$(VERSION_PATCH)' >> $@
 	$(qexec)echo 'Requires:' >> $@
 	$(qexec)echo 'Conflicts:' >> $@
-	$(qexec)echo 'Libs: -L$${libdir} -lvpx' >> $@
+	$(qexec)echo 'Libs: -L$${libdir} -lvpx -lm' >> $@
 	$(qexec)echo 'Cflags: -I$${includedir}' >> $@
 INSTALL-LIBS-yes += $(LIBSUBDIR)/pkgconfig/vpx.pc
 INSTALL_MAPS += $(LIBSUBDIR)/pkgconfig/%.pc %.pc