Fix CMake declaration for absl::str_format

absl/strings/CMakeLists.txt

@@ -82,11 +82,9 @@ absl_library(
 )
 
 # add str_format library
-absl_library(
+absl_header_library(
   TARGET
     absl_str_format
-  SOURCES
-    "str_format.h"
   PUBLIC_LIBRARIES
     str_format_internal
   EXPORT_NAME
@@ -94,7 +92,7 @@ absl_library(
 )
 
 # str_format_internal
- absl_library(
+absl_library(
   TARGET
     str_format_internal
   SOURCES