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/Effects.h

@@ -11,7 +11,7 @@
 #include <LibDSP/ProcessorParameter.h>
 #include <LibDSP/Transport.h>
 
-namespace LibDSP::Effects {
+namespace DSP::Effects {
 
 // A simple digital delay effect using a delay buffer.
 // This is based on Piano's old built-in delay.