Use -g3 rather than -ggdb3 for debian

On a GNU/Linux system -g3 and -ggdb3 should produce the same result
under gcc, but clang only recognizes -g3.

debian/rules

@@ -6,7 +6,7 @@
 
 FS_CC?=gcc
 FS_CXX?=g++
-FS_CFLAGS?=-ggdb3 -O2 -fPIC
+FS_CFLAGS?=-g3 -O2 -fPIC
 FS_CPPFLAGS?=
 FS_CXXFLAGS?=$(FS_CFLAGS)
 export PATH?=/usr/lib/ccache:/usr/sbin:/usr/bin:/sbin:/bin