Fix font-edit linking error: add -lm

The recent refactoring of sfit.c removed the custom sqrt() in favor of
using the standard library's sqrt() and fabs() functions. This requires
linking against the math library.

Link error:
  undefined reference to symbol 'sqrt@@GLIBC_2.2.5'
  DSO missing from command line: libm.so.6

Author: jserv

Makefile

@@ -175,7 +175,8 @@ font-edit_cflags-y := \
 	$(shell sdl2-config --cflags)
 font-edit_ldflags-y := \
 	$(shell pkg-config --libs cairo) \
-	$(shell sdl2-config --libs)
+	$(shell sdl2-config --libs) \
+	-lm
 
 # Headless control tool
 target-$(CONFIG_TOOL_HEADLESS_CTL) += headless-ctl