The tests:
- __support/freelist_heap_fuzz.cpp
- fuzzing/string/strlen_fuzz.cpp
had build failures for different reasons. This patch fixes these
failures.
freelist_heap_fuzz.cpp had this error:
```
llvm-project/libc/fuzzing/__support/freelist_heap_fuzz.cpp:150:26: error: use of undeclared identifier 'Block'; did you mean '__llvm_libc_23_0_0_git::Block'?
150 | size_t alignment = Block::MIN_ALIGN;
| ^~~~~
| __llvm_libc_23_0_0_git::Block
```
The issue stems from the fact that Block was not available in scope. It
needs to be referenced via LIBC_NAMESPACE.
strlen_fuzz.cpp had this error:
```
In file included from Workspace/llvm-project/libc/fuzzing/string/strlen_fuzz.cpp:14:
In file included from /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/cstdint:38:
In file included from /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/x86_64-linux-gnu/c++/13/bits/c++config.h:679:
/usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/x86_64-linux-gnu/c++/13/bits/os_defines.h:44:5: error: function-like macro '__GLIBC_PREREQ' is not defined
44 | #if __GLIBC_PREREQ(2,15) && defined(_GNU_SOURCE)
```
This issue is more cryptic to me, but I managed to fix it by changing
the includes from cstdint and cstring to stdint.h and string.h.
