Git Repository Merge Request #33: Do not set `reset_since_last_plot` if `USE_MOUSE` is not defined (merged)

[a23215] introduced reset_since_last_plot, which broke the compilation with USE_MOUSE undefined.

Reproduction:

./configure --disable-largefile --disable-plugins --disable-x11-mbfonts --disable-x11-external --disable-wxwidgets --disable-stats --without-x --without-libcerf --without-lua --without-tektronix --without-cairo --with-qt=no --without-latex --without-bitmap-terminals --without-gd --without-readline
make

Observed behavior:

unset.c: In function ‘reset_command’:
unset.c:2079:9: error: ‘reset_since_last_plot’ undeclared (first use in this function)
 2079 |         reset_since_last_plot = TRUE;
      |         ^~~~~~~~~~~~~~~~~~~~~