string/strspn/strrcspn.h: assignment discards 'const' qualifier from pointer target type

[zeha]

libtool: compile:  gcc -I. -I.. -I.. -isystem /usr/include/aarch64-linux-gnu/bsd -DLIBBSD_OVERLAY -lsystemd -O2 -Wall -Wextra -Werror=implicit-function-declaration -Werror=implicit-int -Werror=incompatible-pointer-types -Werror=int-conversion -Wno-expansi
on-to-defined -Wno-unknown-attributes -Wno-unknown-warning-option -MT libshadow_la-getdef.lo -MD -MP -MF .deps/libshadow_la-getdef.Tpo -c getdef.c  -fPIC -DPIC -o .libs/libshadow_la-getdef.o
In file included from string/strspn/stprspn.h:13,
                 from getdef.c:36:
./string/strspn/strrspn.h: In function 'strrspn_':
./string/strspn/strrspn.h:29:11: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
   29 |         p = strnul(s);
      |           ^

seen on current master 77eb67d

/cc @alejandro-colomar

[alejandro-colomar]

I'll fix that in a moment. Thanks for the report!

Let's see if we can get rid of all const-related existing diagnostics and then promote it to an error.