LibDSP: Rename library namespace to DSP

That's the standard naming convention, but I didn't follow it when
originally creating LibDSP and nobody corrected me, so here I am one
year later :^)

Userland/Libraries/LibDSP/Processor.h

@@ -17,7 +17,7 @@
 #include <LibDSP/ProcessorParameter.h>
 #include <LibDSP/Transport.h>
 
-namespace LibDSP {
+namespace DSP {
 
 // A processor processes notes or audio into notes or audio. Processors are e.g. samplers, synthesizers, effects, arpeggiators etc.
 class Processor : public RefCounted<Processor> {