strtol.3, strtoul.3: SYNOPSIS: Use 'restrict' in prototypes

strtol.3, strtoul.3: SYNOPSIS: Use 'restrict' in prototypes

Both POSIX and glibc use 'restrict' in strtol(), strtoll(), strtoul(), strtoull().
Let's use it here too.

.../glibc$ grep_glibc_prototype strtol
stdlib/stdlib.h:176:
extern long int strtol (const char *__restrict __nptr,
			char **__restrict __endptr, int __base)
     __THROW __nonnull ((1));
.../glibc$ grep_glibc_prototype strtoll
stdlib/stdlib.h:199:
__extension__
extern long long int strtoll (const char *__restrict __nptr,
			      char **__restrict __endptr, int __base)
     __THROW __nonnull ((1));
.../glibc$ grep_glibc_prototype strtoul
stdlib/stdlib.h:180:
extern unsigned long int strtoul (const char *__restrict __nptr,
				  char **__restrict __endptr, int __base)
     __THROW __nonnull ((1));
.../glibc$ grep_glibc_prototype strtoull
stdlib/stdlib.h:204:
__extension__
extern unsigned long long int strtoull (const char *__restrict __nptr,
					char **__restrict __endptr, int __base)
     __THROW __nonnull ((1));
.../glibc$

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>