[libc] fix getchar_unlocked (llvm#73874)

A typo was leading to getc_unlocked.cpp.o being included into libc.a
twice.

I only noticed because I was trying to convert libc.a to a shared object
via

$ ld.lld -o libc.so --whole-archive libc.a

which errored since getc_unlocked was being defined twice.

Author: nickdesaulniers

libc/src/stdio/generic/CMakeLists.txt

@@ -319,9 +319,9 @@ add_entrypoint_object(
 add_entrypoint_object(
   getchar_unlocked
   SRCS
-    getc_unlocked.cpp
+    getchar_unlocked.cpp
   HDRS
-    ../getc_unlocked.h
+    ../getchar_unlocked.h
   DEPENDS
     libc.src.errno.errno
     libc.include.stdio