relibc: add stdint.h include to sys/types/internal.h

Fixes C compilation failures where signal.h (via signalfd_siginfo struct)
uses uint32_t/int32_t/uint64_t without stdint.h being transitively included.

The include chain signal.h -> sys/types.h -> sys/types/internal.h lacked
stdint.h, causing 'unknown type name' errors for uint32_t, uint8_t, and
uintptr_t in any C package compiling against the relibc sysroot headers.

Affected: diffutils, libiconv, and all packages whose autoconf checks
included <signal.h> and failed on the missing stdint types.

Added 'stdint.h' to sys_includes in sys_types_internal/cbindgen.toml.
Durable in local/patches/relibc/P3-sys-types-stdint-include.patch.

local/patches/relibc/P3-sys-types-stdint-include.patch

@@ -0,0 +1,8 @@
+--- a/src/header/sys_types_internal/cbindgen.toml
++++ b/src/header/sys_types_internal/cbindgen.toml
+@@ -1,4 +1,4 @@
+-sys_includes = ["stddef.h"]
++sys_includes = ["stddef.h", "stdint.h"]
+ # TODO: figure out how to export void* type
+ after_includes = """
+