fix return type error of rt_strcasecmp: rt_uint32_t to rt_int32_t

[leeehui]

Signed-off-by: lihui leehui_hrbeu71102@163.com

拉取/合并请求描述：(PR description)

[
this change is done for fixing the following warning while I execute "scons" under rt-thread/bsp/hifive1
Note the compiler is master of riscv-gnu-toolchain

rt-thread/src/kservice.c:1391:5: warning: 'strcasecmp' alias between functions of incompatible types 'int(const char *, const char *)' and 'rt_uint32_t(const char *, const char *)' {aka 'unsigned int(const char *, const char *)'} [-Wattribute-alias=]
1391 | int strcasecmp(const char *a, const char *b) attribute((weak, alias("rt_strcasecmp")));
| ^~~~~~~~~~
rt-thread/rt-thread/src/kservice.c:373:13: note: aliased declaration here
373 | rt_uint32_t rt_strcasecmp(const char *a, const char *b)
| ^~~~~~~~~~~~~

comiled whith no error and warning with both riscv-gnu-toolchain and arm-none-elf
]

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