[libc] fixup missing include for fullbuild

Fixes llvm#86928

Author: nickdesaulniers

libc/src/stdio/fseeko.h

@@ -10,6 +10,7 @@
 #define LLVM_LIBC_SRC_STDIO_FSEEKO_H
 
 #include <stdio.h>
+#include <unistd.h>
 
 namespace LIBC_NAMESPACE {
 

libc/src/stdio/ftello.h

@@ -10,6 +10,7 @@
 #define LLVM_LIBC_SRC_STDIO_FTELLO_H
 
 #include <stdio.h>
+#include <unistd.h>
 
 namespace LIBC_NAMESPACE {