xkbcomp: Copy the string into the buffer rather than updating pointer

xkbcomp: Copy the string into the buffer rather than updating pointer

Avoids assigning the global pointer to a value that may only have a stack
lifetime:

Fixes valgrind warnings such as:

==24795== Invalid read of size 1
==24795==    at 0x4A06E9A: strcpy (mc_replace_strmem.c:311)
==24795==    by 0x4E54D68: ProcessIncludeFile (misc.c:73)
==24795==    by 0x4E59726: HandleIncludeSymbols.constprop.3 (symbols.c:829)
==24795==    by 0x4E59D8E: HandleSymbolsFile (symbols.c:1673)
==24795==    by 0x4E5A068: CompileSymbols (symbols.c:2211)
==24795==    by 0x4E51A61: CompileKeymap (keymap.c:155)
==24795==    by 0x4E5B410: xkb_compile_keymap_from_components (xkbcomp.c:236)
==24795==    by 0x4E5B587: xkb_compile_keymap_from_rules (xkbcomp.c:161)
==24795==    by 0x405ED2: display_create (window.c:2007)
==24795==    by 0x403732: main (desktop-shell.c:320)
==24795==  Address 0x7fefff0a0 is just below the stack ptr.  To suppress, use:
--workaround-gcc296-bugs=yes
==24795==
==24795== Source and destination overlap in strcpy(0x7fefff430, 0x7fefff430)
==24795==    at 0x4A06F3D: strcpy (mc_replace_strmem.c:311)
==24795==    by 0x4E54D68: ProcessIncludeFile (misc.c:73)
==24795==    by 0x4E59726: HandleIncludeSymbols.constprop.3 (symbols.c:829)
==24795==    by 0x4E59D8E: HandleSymbolsFile (symbols.c:1673)
==24795==    by 0x4E5A068: CompileSymbols (symbols.c:2211)
==24795==    by 0x4E51A61: CompileKeymap (keymap.c:155)
==24795==    by 0x4E5B410: xkb_compile_keymap_from_components (xkbcomp.c:236)
==24795==    by 0x4E5B587: xkb_compile_keymap_from_rules (xkbcomp.c:161)
==24795==    by 0x405ED2: display_create (window.c:2007)
==24795==    by 0x403732: main (desktop-shell.c:320)