Fix compilation warnings

Only visible in release mode with default set of compilation warnings enabled, i.e.

CFLAGS=$XFCE_CFLAGS ./autogen.sh --disable-debug && make

with

XFCE_CFLAGS=$(echo \
  -DXFCE_DISABLE_DEPRECATED \
  -Wall -Wextra \
  -Wno-missing-field-initializers \
  -Wno-unused-parameter -Wold-style-definition \
  -Wdeclaration-after-statement \
  -Wmissing-declarations \
  -Wmissing-noreturn -Wpointer-arith \
  -Wcast-align -Wformat -Wformat-security -Wformat-y2k \
  -Winit-self -Wmissing-include-dirs -Wundef \
  -Wnested-externs -Wredundant-decls -Wshadow \
)