Explicity include string.h from pgmspace.h

Author: ianfixes

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 - Replaced pipes with `Open3.capture3` to avoid deadlocks when commands have too much output
 - `ci_config.rb` now returns empty arrays (instead of nil) for undefined config keys
+- `pgmspace.h` explictly includes `<string.h>`
 
 ### Security
 

cpp/arduino/avr/pgmspace.h

@@ -16,6 +16,7 @@ out.each { |l| puts d(l) }
 */
 
 #include <avr/io.h>
+#include <string>
 
 #define PROGMEM